Ok I'm well aware of the rule that says a guy can't go out with his friend's ex-girlfriend. But does that rule apply when the girl in question has cut out the other guy from her life entirely?

Your alliance is to your friend so make sure he's OK with it. I always say it's not OK to date friends' Ex's but a few people are OK with it. Talk it over with your friend first.

You have to be ready to gamble the friendship

And odds are you'r going to loose a friend

Why not just ask him, then you won't have to guess what his rules are.

How close are you with your friend and how serious are you about his ex?