Should my friend pursue custody at this time?

my friend has 3 children with this guy, he's on the 2 oldest's birth certificates, but not the youngest. well he was hitting her and cheating on her and going out and doing drugs, leaving her to take care of the kids. One day, a couple of years ago, she finally left, but the father and his family wouldn't let her take her 3 children. She was afraid to take them because she didn't know if the law would be on her side. So she went back and visited with her kids, when she could or when they would let her, and eventually they quit letting her see her kids (about a year ago) and the father went to put her on child support. She found out, recently, that he signed a waiver of services though...not sure what that means...and he also just recently went to jail because of a couple of dui on his record...She wants to know if she should take this opportunity to pursue the custody of her 3 children, while he's not there to hurt her or stop her. And if she does, how does she go about doing it?