Thanks for all the comments, everyone.
It really is a messy situation, right now. J is feeling very caught. Their relationship has been rocky for a few months, partly due to a VERY minor mess up on her part, but his reaction has been way out of proportion and has led to a lot of difficulties. She's really starting to see that, and we had a very good private talk last night after the 11 hour talk between the three of us. She feels she needs to try this one last stab at seeing if he's willing and able to work on their relationship. I'm supportive of her, but I don't have any faith in his ability to be anything other than a selfish manipulator.
I'm doing everything I can to keep her from being isolated by him. I know that she and I will be able to keep ourselves intact through this. I actually feel for him, since he's sabotaging himself and doesn't see how. I don't feel TOO bad for him, though. He's making his own bed.