Your H should've thought to ask D13 bring a friend (and allowed it, once she asked), but there ya are ... he's not thinking about anyone else, but himself. Even though this seems like he's doing this wonderful thing for his mum, and he's all lovey dovey with the OW .... ultimately, it's all about him, and what he wants at this moment in time. All you can do, is keep out of his way as much as possible, while still protecting yourself and your children legally and financially. Just my opinion, of course.
I hope D13 had a good time despite all this.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim