My fiance ex wife is living with an abusive boyfriend.can he keep the kids from her until court?

They currently share custody 50/50 of their 3kids 10, 8 and 4years old. The ex wife stays with her parents on her weeks with the kids and then goes back to the boyfriend who lives over 100miles away. My fiance is planning on going back to court to either get full custody or get it so the kids can't be around the boyfriend as he feels they would be in danger. The boyfriend is a convicted ex felon and has had the police called to his house for domestic violence. My fiance has told the ex wife he doesn't want his kids around the boyfriend but she said she doesn't care. Summer vacation is coming and she plans on spending her weeks with the kids at the boyfriends house 100miles away. She has had over a dozen parenting time violations and it is also at the point where my fiance has the kids 80% of the time since she doesn't always come on her week or shows up 3days late. She has made it clear that she plans on taking the kids for the weekend to go camping with the boyfriend and this is after she never showed up for her week with the kids last time because she got beat up so badly. The kids have told my fiance that they've witnessed the boyfriend pushing their mother around on numerous occasion's. My fiance has called and left messages for his case worker but with no response. Would my fiance be in trouble if he decided to keep the kids from her this weekend since she has said that she doesn't care if he's not comfortable with the kids being around her boyfriend, if he's planning on filing a motion with the courts this week?