FB Friending Dilemma

I have a friending dilemma on my Facebook account. There is a girl who I do know in real life and I like her. But she is friends with my ex. I don’t like the idea of my ex possibly using this friend to stalk me on Facebook. So I don’t really want to friend her, but I don’t know if this is a real concern of just me being paranoid.

My other friends say that she is in touch with my ex and although no one has told me that she is dating him, I would not be surprised. So I have to figure out how to find out if she is really friends with him or dating him or what. And why this sudden interest in being Facebook friends?