dude, don't tell him with the gf present...

Am I the only one that thought of the song "my best friends girlfriend"

Bros before Hos, anytime, any place. You break that code, you ain't Bros no more.

As someone said above, I guess he had to choose sides at that point and he chose to believe his brother and gf over you and that sucks. He treated you badly. But if he had said he needed time to think about it or something, with the gf there that could have led to some serious problems if she hadn't been cheating. I think you did the right thing you have definitely been treated very unfairly, but trying to see it from his perspective, he has to accept someone is lying to him and he didn't have space to think it through at the time with his girlfriend in the room, possibly saying 'you can't believe him'.

Well, you tried to warning him. If he doesn't believe you, it's his lost, OP.

You should of told him privately. He's not your best friend if he reacts in that way

Best case scenario: your best friend uses FML and sees this post

Yeah thsts exactly what happens in those situations. Sometimes they turn violent. You'll know better next time.

Don't worry-your words will sink in, and he'll be watching her. They probably had a fight right after you left.

Obviously spellbound to the power of her body. At least you tried, stick by them when it falls.