Well, all I can tell is of one experience I had back in University - freshman year.
I was dating my then GF (now wife), but I was in Europe for a semester. Also i Europe that semester was the GF of my best friend (and about 60 other students). Now, I really didn't know her but would say "Hi" as we passed in the halls, etc.
So the semester passes and I come back to find that my friend wants to kick my @ss for trying to steal my GF and he admits that in retaliation he tried to steal mine! (He got nowhere he admitted).
So we are furious at each other for a while and I can't figure out why he would think that I was trying to steal this girl I didn't even know!
Until I later learn from one of his GF's friends that she had wanted to break up with my friend but didn't know how, so she started writing him and cakking him telling him that she and I were together! :Q
Again, let me reiterate that I barely said "Hi" to this girl the whole time in Europe!
She finally told my friend everything and they broke up - he was really sorry he believed her and we patched things up as best we could. I was annoyed to have been unknowingly used as a weapon in thier breakup liek that, but I finally knew why at least.
Not sure if any such thing is going on with this friend of yours, but I'm trying to point out that a lot more might be going on than you know about. Try honestly talking to your buddy (without dissing his GF!).