This is what I was going to say.
I've been in the "It was only a kiss" situation before. Eventually I got all the info out of my exgirlfriend. It ended up being more than simply a kiss.
Now, if someone keeps a kiss from you, that's the same as lieing. If they can lie about that, who's to say she isn't lieing about more?
Sorry, I'm not trying to bring you down or add to your hurt, but I've been there before and sometimes people don't tell the WHOLE truth, only partial truth.
Either way your friend & GF betrayed you. Kick him out and then rethink what she has done. Can you really trust her honesty now or not is the question you should be asking yourself.