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 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.

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."

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

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.

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...

If she is a 'friend'...

... 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

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.

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

All very bad ideas from snail mail on down

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.