Hello, maybe what I'm going to say is quite rude.. and you might say I don't understand, it never happened to me..

I think you're overreacting.. You had a very short relationship, you shouldn't try to stop love, if they've been toghether for so long they really have something... As a friend, you should understand that. 

And even if you can't forgive that... Everybody has different ways to manage in life, you shouldn't think all of us are bad people only because you think your friend was.