Support?

My fiancé says I never support him when it comes to his mother. They get bad to the point of screaming and grabbing when she is drunk, over her not leaving us alone. I've been around drunks my entire life, so I handle it a lot better than him. When she's drunk he doesn't want her around and neither do I, yet I try to play nice to get her to go away he just gets mean and yells. Then he gets mad when I don't back up him when he starts fighting with her about leaving. I don't what to pick a fight with her or him, and I've told him how to handle it better, yet in the end we end up fighting. How? Or what am I to do?