Sasha, So I have a problem and it's to do with my very best friend.  She's not a monogamous person, but she has a serious boyfriend right now who is.  They have been together for about a year and a half and are planning on moving in together.  He is wonderful to her.  He loves her dearly, treats her really well, etc etc and wants to marry her.  She loves him as well, but she cheats on him.  She has a guy who she sees a couple times a week at least and there are random men here and there as well.  And her boyfriend has no clue this is happening.
When I talk to her about it she says that her boyfriend can't give her what she needs in bed.  He is caring and loving, and she wants to be dominated.  And the few times they've tried she can't take him seriously because he's not physically dominating in any sense.  So I've been trying to push her to break up with her boyfriend because obviously they're not the right fit.  Or stay with him and stop her dalliances.  Or at least cut down her dalliances.  Or change something.  Anything.  But nothing sticks.   Her boyfriend is a good person and deserves to have a proper relationship.  But I want my friend to be happy as well.  What do I do? S

Yikes, your girl is being hella shady, but here’s the good news S, you’ve done everything right up to this point – you’ve been a stand-up person and friend through this.   But now you’ve got to tap out of this drama.

If your best friend wants to be a lying sack of sh-t, let her be.  You’ve tried your best to persuade her to be an honorable human being and she’s not listening, so it’s time to drop the issue.  The next time she tries to bring up the issue with you, be clear with her that you’re not interested in talking about her cheating ass.  The end.  Next topic.  

Oh, but this poor dude.  Ugh. I feel so bad for him and I know you do too, but from what I sense you guys aren’t super tight, right?  And if that’s the case, if you guys aren’t buds independently of her,  I really don’t think it’s your place to say anything to him.  At the end of the day it isn’t any of your business.  I never thought I’d have to say this, but hoes before bros. 

Here’s the one caveat though, if you find out that your girl is having unsafe sex with her side dish then all bets are off –you have no choice but to hang her ass out to dry.