How to handle jealousy, in this situation.?

Here is the situation. I met this girl about 4 months ago and we have been dating. I really like this girl and I am trying to trust her, but there is this one situation that keeps eating at my mind. She has this friend that she has known for like 6 years, it's a guy of course and they were intimate at one point in their lives. This happened like a year before I met her and she claims it was dumb drunken and the guy is too old 35-23. So I figure that's cool, we have all made similar mistakes. The problem is they are still really close and I can't tell if they are just good friends or if I should be worried. And it constantly makes me jealous. Now it's getting to the point were I start thinking things and getting bitter. How should I handle this situation without over stepping my boundaries. And is it normal for "platonic" friends to spend so much time together and stay at each others houses. Help from either gender would be greatly appreciated. I am very understanding. Thanks