i had that happen 2 me, my best mate went out with my x, they hooked up at my bday party, and althought it was weird im cool with it coz thers so right for each other, but it would have been nice if one of them had said 'i think this is going to happen, do u mind?' i think u should ask ur mate, maybe wait till he's seeing someone else so u know there cant be any feelings there for the woman in question, and then ask him. do it when ur drunk so u can blame it on the booze if he hates the idea.