Whatever she may say, the problems between him and her are not about you. She may find you an easy scapegoat....you may be the subject of their first few fights....but eventually you'll find you come up less and less....or maybe you don't.
Either way, what's really bothering her is something else. I promise you.
The best thing you can do, most likely, is be supportive-- of both of them!-- but at the same time, make it clear that you are not going to get involved in their issues. No need to be antagonistic...just firm. Don't take anything she says personally (or he says? you make it sound like she's the one who has taken issue with you).
Let them work their relationship out, while finding a way to maintain your own sanity and your own relationships with either or both of them