This is your best friend in the world, right? If I were you, I'd have a heart-to-heart talk with her and tell her that you love her to pieces, but you simply cannot be around her boyfriend anymore and WHY. She may not be excited to hear this, but I think you owe it to her to be honest. And then give the jerk the cut direct. If she chooses to stay with him, there's not much you can do about it. But your honesty will give her something to chew on and maybe she'll figure out that he's costing her valuable friendships and dump him.
Saw your update, but thought I'd put my 2 cents in anyways
I'm with Shoo on this one. This isn't a small thing that won't affect your relationship with her if you keep quiet, but might hurt her feelings if you tell - not telling is going to drive a wedge between you anyways because this guy is going to either push you over the edge or you'll start having to avoid her to keep that from happening. If this is your BFF, how is she not noticing how he treats you? How have you gone this long without discussing what her SO is like? When we date people, my friends and I usually talk about that stuff. My BFF had no problem saying that she was sure there were things that made him more attractive to me as his date, but as just an acquaintance of his, she found a guy I was seeing to be arrogant. I understood that this was her being my friend and being honest, and not her trying to upset me.
To keep a long story as short as possible, I was living a long way away when they were building their relationship
. Through phone contact etc I did make it clear that I didn't like the sound of this new guy, but it was based on his reported treatment of HER without any personal feeling involved at all. By the time I moved back close enough she felt that she had worked through their issues (I don't think he's improved as much as she thinks, but that's another story) and the relationship
was too far progressed for me to be stating personal likes/dislikes - as in, they have a child together and live together, so waaaaaay too late for "Are you sure he's the right one? Seems kind of a jerk to me."
So she knows that I don't entirely approve
of him, but she also knows that I won't be getting in her face about it and am just there to support her when she needs it. And to be honest, the fact that I think he's a jerk to HER far overshadows that I think he's a jerk to me, which is probably why I've let this get this far and have never put a personal slant on it with her.
Actually now that I type it out, that is probably the main reason I have neglected to act on this earlier. She is one of those friends that you can discuss anything with with perfect trust - when DH and I were struggling with a few things she was literally the only person that I spoke honestly about it to, knowing that it would never, ever go any further and also I could trust her not to let it affect her behaviour towards DH, making it socially awkward when we sorted things out. Maybe I just feel that I need to do the same thing for her - be the safe person that she can say things to without it causing complications elsewhere. And she can, I just have to work out how to not translate that into letting him be rude to ME.
Thanks for asking that question! Sorry for the looooong reply, but it's really helped me sort out some things in my head.