Write: I have been reading this thread, but I am baffled, really. Relationships are just so sticky.
I have been where your friend is. When I got involved with my exN, the father of my children, I was told repeatedly what a shit he is, etc.....but for many reasons (that took years of therapy to conclude) I went ahead and married him anyway....
My sisters who did not like him at all, for the way he treated me and everyone else, basically shut up and were kind to me for 13 years...must have been tough for them. When the blinders came off and I decided to divorce the cheating NNNNNNastiness, my sisters were right there behind me. Not really telling me "I told you so" but really, they didn't have to say it. They just supported me.
I don't know if they refused to do anything with me because of him... but I'll bet they did...politely declining, etc. Helps that we lived very far apart, though.
I am surprised they came back to my rescue though. Wouldn't blame them if they wrote me off.
I think you are on the right track, though. Don't do anything that violates your own boundaries. Be there when and if she needs you. I think that sums it up.
This stuff still confounds me though. Maybe I'll pick it up on another thread.