DH, Lots of times people play really hard when it comes to divorce. I met her once, and know some of her from postings she's made, blog and FB, and mutual friends, and she seems to be nothing but really nice. She seems to be highly regarded in my local area as a poly person/person in general.
I don't think there is any reason for his behavior other than the typical "Now that we are divorcing I hate you and will make your life hell just because I hate you" I have seen plenty of that over the last 20 years, so just because it makes no sense to cause misery for no reason, doesn't mean it doesn't happen every day, in hundreds of divorces, mono or not.
Wish I'd gone to law school so I could offer some assistance in her case. Her situation was actually a topic of my husband and I at dinner tonight.