This is a tricky situation... Only once have I ever gone after a friend's ex/gf/bf/sig. other, but there were weird circumstances. Generally, i think it's a big no no. If 2 friends find themselves attracted to the same person, the friendship should come first and they should BOTH remove the other person from their lives. If one friend continues to persue the other person despite knowing it could hurt their friend, then maybe the friendship wasn't as strong as they thought.

Friendships should ALWAYS come first. Friends will be there for the long haul, but "relationships" will come and go. Obviously in a strong friendship hurting each other, stepping on each other's toes, shoudlnt even be an option. It would be silly to ruin a friendship over a guy, especially when the chances of the relationship being long term are slim! But at the same time, if your friend doesn't respect you enough to lay off maybe the friendship should be reevaluated.

Just my 2 cents.