Tax Refund Claiming The Kids

Legally in the divorce decree it says that my girlfriend can claim one and her ex husband can claim the other child. However she found out that he did not claim the child he was allowed and she already got the money from support when they took his tax refund. So what I'm asking is if it is legal for her to amend her taxes and claim both kids with telling anyone?

No.

She's bound by the terms of the decree, just as he is. If he's dumb and forgot, that doesn't give her the right to violate the decree.