She can still get a divorce even if he decides not to sign. She may actually be able to get an annulment based on his mental status, or physical abuse...it all depends on your state laws and what reasons qualify for an annulment.
I agree, she needs to get out IMMEDIATELY. My husband is BiPolar and although he's stable & on medication, they can easily fly off the handle, especially when not monitored or on meds. He can really hurt her at the drop of a hat if something clicked wrong in his head. She needs to leave immediately and get the process started like the others said. Make sure her & the baby are safe, then once the baby is born the divorce can continue.
Best of luck, get her out of there ok?