That's the catch. I could see MOMO getting involved if she thinks people who aren't involved might get hurt. If Fou-Lu goes to ask her for help and outright says, "Yeah, we're gonna kill 'im," she might not be so willing. Not that she hasn't killed, but she's only done that for the preservation of the greater good. Fou-Lu's going to either have to be a really smooth talker to get her in on it, or she'll have to come by having detected the fight somehow and moves in to protect someone... Like if she sees Xellos making royal damage to the party, she'll help against him.