Sean Hollyman said:
What would you do in this situation, if it happened?
Basically:
You find out that your best friend's girlfriend, is cheating on him, with his best friend, who is also your other best friend?
Would you tell him, and risk ruining both your friendships, or let it carry on?
Of course you tell him. Tell the other one, too. If your friends aren't mature enough to have a frank discussion about the love triangle going on, why are they your friends again?
Your friend deserves to know what's going on, and your other friend deserves to catch shit for being a dumbass. The girl will probably get dumped (she deserves it), there will be a fight, and tell both friends you're staying out of this.
If the friend who cheated with the other's girlfriend accuses you of betraying him, remind him that it was his actions which got him caught and that you would have told him if his girlfriend was cheating on him.
If the friend being cheated on accuses you if witholding information (not likely), then remind him that it wasn't an easy position to be in and you did the right thing in the end.
People will ***** and moan for a while, and anyone who holds a grudge against you isn't worth being friends with. Period. You don't need friends who don't respect your moral choices and choose to shift the blame onto you than take responsibility for themselves.
The only exceptions are:
1) If you're in your teens, because relationship drama is what keeps all teenagers from commiting suicide out of boredom.
2) If the couple has an open relationship and everyone knows what's happening. It happens, and your person opinion about open relationships aside, it's your job to respect their beliefs.