I'm not exactly a newbie, but figured it might be worth asking here....

Be a cut off point for me

One is any behavior that leads me to believe she's not safe to be around.  

One of my regulars got locked up on an assault charge...she used her one phone call to call me at 3am...that ended that...

My first question to the relative is why aren't you bailing her out of jail yourself? If that provider doesn't have the decency to make the call to me directly she is on her own.  

Basically doing anything to inveigle her personal life with yours is plenty reason enough.

over 6 years.  I'd been present through many drama incidents with others.  It made me uncomfortable when I really wanted to be discreet.  In every case, I tried to be a friend & calm her down because life happens.  
Then she allowed me to book a date even though the timing was really bad... a major event in her family was dumped in her lap to sort out with just a week's notice.  I didn't know, because she didn't tell me.  Was she that hard up for money?  It should have been postponed or at least tightly scheduled (we had a no time limit plan in place & I bought generous lunch fixings because I needed to eat so we typically made a nice time of it.)  Well, she stressed out & went off on me when I called after shopping & again when I arrived.  If I hadn't promised I was coming, I'd have turned around when she started in again on my arrival.  The date went forward because again, I hadn't put the donation down.  
She knew she'd crossed the line.  An appology could have fixed it.  No appology was forthcoming.  

I actually had something similar happen to me. it wasn't a provider's friend asking for bail money for the provider. It was a provider I had seen a few times and was friendly with (Knew her real name and personal info etc.), who was asking for money to bail out her friend. i actually sent the money because of the type of relationship we had. In most cases though I would not

For me, I would add:
She moved
Her rates increased above my budget
Her looks changed drastically (i.e. Gained a lot of weight)
She starts treating me like an ATM.

Good things tend to happen.

The same thing happened to me, and I bailed her out; any "paper trail" never entered into my decision.  

Depends on your situation at home and how well you know her personally.

So, I'm still unsure how much more I will see her, but this situation specifically wasn't it for me.  For me, there really isn't much risk to my personal or professional life by continuing to see her.  As strange as it was to get a call from her family, and hearing this family member give her "working" name, since I didn't really recognize her real name (!), I can deal with that.  She isn't a bad person or treating me like an ATM...at least, not excessively so!  And I really do enjoy our sessions together.  Oh, and ultimately, she ended up being released a couple of hours later without bail.  Even if I had posted bail (something I was prepared to do), it would have been via money order, and the paper trail would have been tenuous at best.  As I mentioned, her arrest was NOT hobby related.

In this case, the relative told me she just didn't have the money.  I'm guessing the provider used her one call (do inmates really only get *one* call?) to contact this relative, and when they said they couldn't help, the provider suggested to contact me.

Her being detained and all that, especially if she was released hours after she contacted you for monetary support.  

One has to be prepared to politely say no when it comes to the off the clock favors.  I had a provider try something similar before but I sort of could tell that she was going to try and cross the line so when she asked me for the favor she backed off and I could tell that she was brought back into reality when I said no.  If I had said yes, I would have been opening a door that would have led to more "favors".   Learn to politely say no.

Since it was bail, giving her money directly wouldn't work (county doesn't take cash, only money orders or cashiers checks made out to them).