It's really hard to sit and listen to someone tell lies about your friend and still be professional, while not agreeing too much

I know what you mean. Not on a professional level, but on a personal level I keep ending up being friends with people who hate each other.

Sometimes you should pick sides and not be a people pleaser. Use him and once it is over and he has no power over the efficiency of your work, go back and tell him he is a pos.