Man oh man, this sounds really messed up, the stuff of films...

For a start, I think you did well in asking her to leave your house, she knows now what a stupid mistake she has made by 'allegedly' bunking up with your so called best friend! I'm just wondering why she told you in the first place. Did she really think that you'd just forgive her? You're obviously not that kind of person, and that's definately a good thing! Another question, is she still involved with this guy? Or was it just for sex? Sorry to be so blunt but I think it's important. Is your 'best friend' hiding a secret from you, maybe he and his wife are swingers or something!! The fact that they completely denied all knowledge is quite disgusting. What has his wife said, is she not surprised about the accusations?!

Anyway, sorry I'm not much help, just some things to think about.