Is it wrong that the new girl I'm dating is friends with some of my ex's friends?
She said she's not happy because I'm rubbing her nose in it. No one has said anything to her and I haven't seen my ex.
Depends on the situation, but in your case: no, it does not seem wrong at all.
With BPD there is a risk however... she just might try to influence your new girlfriend directly or indirectly.
I seem to have a lost a couple of friends that way. :-( (but then... I've chosen not to tell the ugly stuff about her. Although chivalrous and all, it sure doesn't help me)