Depending on the jurisdiction, and if the lady has committed a felony sex crime, most states would deem any warning to the lady be the crime of hindering prosecution:

(a) A person commits the crime of hindering prosecution in the first degree if the person renders assistance to a person who has committed a crime punishable as a felony with intent to

 (1) hinder the apprehension, prosecution, conviction, or punishment of that person; or

 (2) assist that person in profiting or benefiting from the commission of the crime.

 (b) For purposes of this section, a person "renders assistance" to another if the person

 (1) harbors or conceals the other person;

 (2) warns the other person of impending discovery or apprehension;

 (3) provides or aids in providing the other person with money,
transportation, a dangerous instrument, a disguise, or other means of avoiding discovery or apprehension;

 (4) prevents or obstructs, by means of force, threat, or deception,anyone from performing an act which might aid in the discovery or apprehension of the other person;

 (5) suppresses by an act of concealment, alteration, or destruction physical evidence which might aid in the discovery or apprehension of the other person; or

 (6) aids the other person in securing or protecting the proceeds of the crime.

 (c) Hindering prosecution in the first degree is a class C felony.

and I'll not disagree.  I don't know the source of your information but if you pass the information on, you might put not only yourself but also the source at risk.   As much as you want to help...  and I would want to also...  your first responsibility is to yourself.  

Does she perchance have an account on a review site where you could send a PM??  (Preferably an offshore based site.)  Just a thought.
I don't know enough to know how to beat the system on e-mail...  perhaps setting up a free e-mail account from a public network (library/coffee shop), though I'm not sure if the individual laptop could be traced.    I don't know enough about proxy servers either...  but this is getting complicated.  If she's in this business she's probably got buckets of messages...  yours would stand out though if LE figured out that it was a warning.  

How about an el-cheapo throw away phone?  Use it once & dump it.   Or find a rare pay phone.    

Sending a coded message through the mail assuming you have snail mail address relies on the recipient being able to understand the code.

These are not the question you asked.  You asked the possible consequences of giving warning.   There is a risk to you.  You might not even get prosecuted...  but you might make yourself into a target or implicate yourself as a participant in this criminal enterprise.  

Personally, I would try to give a warning in such a way that you minimize the risk to yourself...  It's easy for me to be brave since my arse is not at risk.
skb

Ok, I find myself in a peculiar situation.  I happen to be friends with someone who is running an escort agency.  I recently found out that she has become the target of an investigation by vice, and they are trying to build a case against her.  I'm pretty sure she is being watched, that her email is read by vice, and her phones are tapped.

Do I warn her?  

What kind of trouble can I get into if I do?

How do I warn her in such a way that she gets the message but that it doesn't appear to come from me?  I've thought about using one of those anonymous email services but, even if they are truly anonymous and untraceable, how would she know the message is from me considering it's anonymous?  And wouldn't the email likely get deleted as spam anyway?

Or, do I run away from the situation and let her get arrested?

What would you do?

I always warn people to be careful of alerts they give, especially pertaining to LE.
I know you want to be a good friend, but what is a warning going to do. If she has landed herself on the vice list they will find a way to get her. What she needs now is a good lawyer so all she gets stuck with in the end is tax evasion and some slap on the wrist promotion charge. The Queen of all Madame's, Heidi Fleiss only served 3 years

(And I can appreciate the way you feel.)

I think that the safest thing to do is to send her a letter that contains some form of information that seems innocent enough by lets her know who sent the letter, and warn her in that letter of what you know.

Do not post mark it from any area you live or work near, and of course, no signature or return address.

Hope she doesn't decide to play ball with LE and use the letter against you.

Also, the letter should not be handwritten, of course.  Get it printed on some printer at a Kinkos or simiar place.  Have the envelope also printed there.

I think that is about as safe as you can be.

Could you contact a lawyer you know and tell them what you know about how the escort service person is being watched, and then ask the lawyer to contact this person and inform them of this fact, relying on the lawyer/client privilege to shield all parties from prosecution should LE become aware of these conversation?

(still not a lawyer)

You should handle this letter with gloves at all times.

and:

(still not a lawyer)

usually a misdemeanor, usually applied when police officer's directive is not obeyed as in crowd control but might be applied here as well.

It is clear that one should not hinder or interfere and that one does so under one's own peril. In cases where one decides to defy such warnings, person to person warnings in a low voice in a public place are usually the means of communication; however, it is unlikely that a person under surveillance is unaware and to further complicate the matter may be by this point cooperating with the police. Therefore, best to stand away unless one values the friendship more than the consequences of acting.

Was it Dante who said, "there is a special place in hell for the noble heart."

attorney client privilege might not apply, it requires that the the other party was a client at the time the  information was conveyed

How about, purchasing a new untraceable phone, and send her an SMS, don't identify your self but include enough information about her so she will know it is from someone that knows her.

Afterwords, dispose of the phone appropriately.

I doubt her US Postal Service mail is being monitored.  Send her a somewhat anonymous letter.  You can mail it from a location not in your zip code and include some kind of statement, like describing something about the last time you meet that only she would know, in order to identify yourself to her.

go on a weekend vacation
to another continent

tell her to pack ALOT

And I hear what everyone is saying.  I certainly don't want to get in trouble with the police for obvious reasons.

But, on the other hand, if I know and don't tell her, and she finds out that I knew and didn't tell her, I lost a friend.

It's a tough situation... and I'm understandably torn...

... a face-to-face meeting in a neutral, private place where you can tell her what you know would be best, IMHO.  This place needs to be someplace you can be certain is not 'bugged.'  In your own vehilce and driving down the highway comes to mind.  The only remaining risk to you would be that SHE had already been flipped and was wired - and I think that is stretching the bounds of credulity a bit.  

Sure, you can go through all the James Bond stuff to anonymously mail her a letter that cannot be traced back to you.  But why would she believe it?  Or perhaps it would not even be opened given the lack of return address etc.

Or you can walk away.  She knew the risks when she went into her business.

MVR

I would go three steps further, get the prepaid phone( avoid email at all costs) track phone does not require personal info. It may try to ask for it during activation, stay on the line and it will bypass. Then send an sms through a text spoofing service. There are many free ones out there, but don't use google to find it from your personal pc. Go use a public computer at a library or college campus.

Above all, she really needs an attorney that is hobby case friendly

go buy kites, find a field where you can launch them up and watch for the message

go to a basket ball game together, buy one of those fingers, arrange to have the message flash on the flashy message thing, point your finger at her significantly for a minute, then at everyone else, but not at yourself  

scribble it on your naked asschecks, jet out to paris for a romantic getaway, tell her to pack ALOT,  go commando, moon her midflight  

get the homeless dude to write a message and put it into his wino bottle, arrange for him to throw it at her as you guys walk past,.   let her pick it up... act shocked and dismayed... run.

keep 2 packs of hubba bubba bubble gum with you at all times in case you get questioned,  stuff a wad in your mouth before you tell them your name and you address . ..after they got that, stick a wad of gum into each ear canal... now you can concentrate on the very important task of of blowing 1 million bubbles.. .. out your ears, you only have to do half..

The cloak of anonymity that TER gives should hep.

Also, the prepaid bat phone works too.

Don't idenify yourself. Tell her what you know and hang up. You have plasuable deniability if ever needed and the best she could say is I think it was so and so. LE won't spend 2 min. looking for you. They will spend days trying to figure where the leak is.

old school...  low tech...  simple.  That's why it's great.  It might take some doing to find the right one but paying cash for the call makes it untraceable, Clark Kent.   There's still a pay phone outside my local supermarket.
I'd open with "I'm a friend, if you recognize my voice please don't say my name."  "I have reason to believe you are under survailance...  "  
If her phone is really tapped, they can follow it back to the empty phone booth in another jurisdiction...  LOL!!  
skb

Once she is arrested they will offer to plead her down, or give her immunity, for cooperation if they are after bigger fish.

And the first question they ask her is-

"who tipped you off"


      Just don't do it, dude. Never interfere with an on-going police investigation.

being thrown at her by a homeless dude ?

it's a tad unlikely.

althogh i admit mooning the message in midflight would probably be tracked back to him, it's still my fave.