[Morgeruat]

hey, I know exactly how you feel, I went through the same type of thing, twice, not quite as serious though, but it ended up destroying the friendship between me and one of my best friends, I can't stand the guy anymore, my advice is to tell him how you feel, and that it is very uncomfortable for you, it probably won't change anything if he's determined to see her, but you should come to some sort of agreement, like letting you know when/if she'll be around so you can avoid the situation, and if it is still too fresh a wound you could ask him not to discuss the relationship with you at all.

And feelings that run deep are hard to destroy, some are impossible, what worked for me the first time my roommate did this was I quit talking to the girl, and have him not discuss the relationship.