I don't know if its good or bad that I have never had to experience anything like this but it sounds interesting. I would be pissed if my roommate was getting ready to hook up with a girl that I had been seeing, even if the relationship was going nowhere...just respect.
I had a guy that I hung out with (hated him but he let us go to the beach for free) and he had a girlfriend, fucked a girl that was down at the beach that was friends with me and my friends and then was talking to my now wife....all in the same weekend. I found out about it one night when his girlfriend called and I answered because he was "busy" and I talked to her for a good while and she told me she was his girlfriend. So my friends and I decided that we would fuck his house up (dumb on our part, but we were young and had been drinking for 3 days straight) we glued all his clothes to the floor, all his money we glued to his nightstand, glued all the light switches on, glued just really fucked this dudes house up.
he was passed out, we all went to sleep. woke up in the middle of the night, well like 5 in the morning to the dude screaming and yelling and wanting to fight us for all the shit we did to his house. we had out words, I told him that I told his GF about his little mishap, we packed up our stuff and left.
Thinking back....fuck that dude and he had the most stupid laugh I ever heard in my life to this day.
So I guess what I am trying to say is, best friend, roommate, or just friend...still fucked up to take a girl from your bro.