Facebook is really the devil with situations like this. I am friends with my ex-husband, but only so he can see pics of our kids. I've set up security so he can't see any pictures I'm tagged in, and the ONLY pics he can see are the ones where I specifically tag his name. He also can't see my wall, so he can't see what anyone else posts, only when I *occasionally* post a status or a link for ALL my friends to see can he see it. It makes my kids feel good to know that he can see their pictures on there, that's why I'm still friends with him, and I have him hidden from my news feed so I don't have to see any of his stuff, either. My kids are 6 and 9, too young for their own facebook accounts to keep in touch with their dad on their own this way but when they can, I will unfriend him.
Anyway, I highly recommend either unfriending or changing your security settings!