Here's the full story, as far as I know and for the record, I have absolutely zero involvement, aside from knowing about it.
She moves out, takes the kids with her.
A week later she comes by on the pretense of retrieving belongings. While she's there she takes a check out his checkbook. Fills it out, signs his name and cashes it. No idea what it was spent on.
Later that night or the next day she messages him informing him of the theft.
He chooses not to report it, for his daughter's sake.
A few weeks later, after not having seen his kids in a week he goes to visit. She tells him to leave, he leaves without incident.
The next day he receives a FPO, where she claims he's violent and unstable.
Meanwhile, he's been paying for the kids daycare ($600month), buying them groceries, paid for mom's child car insurance.
She left him with lots of bills to pay. Took her phone off the family plan and left him with a $400 cell phone bill to pay alone (she has a job).
She's been going out bar hopping/partying every weekend. Called him one night and told him she was taking some guy home.
Bear in mind, her bank account is overdrawn, her car is close to being repossessed etc. etc. and she's going out bar hopping every weekend.
So, forgery aside - what is he supposed to do now?
He can't even afford a lawyer right now. He's working full time and going to college part time.