Firstly and most importantly - do not in any way threaten, stalk, or cut the heads from her photographs and post them through the mailbox. That way lies restraining orders and late night visits to the police station.

This is a touchy area to deal with. If - as in my case - your Straight Co-Star and his girlfriend are already having problems it's not all that difficult. Make sure you're available for him. When he claims that she doesn't listen to him, be there in her place. Avoid calling her names as it's likely to bring out his protective instincts - he does still care for her, after all - and simply try to show that whenever she isn't there for him, you are.

Sneaking around behind their backs and trying to set her up (with another lover, stealing money from his bank account, developing some kind of STD, etc) is not the thing to do. It's just not classy, and could end up backfiring if they ever find out.

If, on the other hand, there are no obvious cracks in their relationship, you do whatever it takes to get the crazy bitch out of the way.