Can a mother refuse a father to see his child if the father has joint custody?

My husband has joint custody with his ex wife and she refuses to let the kids come back to our home. She just filed a false case child abuse and it was NOT founded. We are foster parents and if we was abusing children don't you think the social workers would take our license? We have social workers, guardian item coming to our home every week. Don't you think they would sense abuse in our home. She pays child support and she asked many times can he stop it. i feel that is the main problem. I feel shes trying to get a case on him so she can get the kids but she's never filed any paperwork with the courts stating that she want the kids. Just the child abuse case with DSS. My question: Can she refuse the father to see the kids? He's suppose to have the kids mon-fri and she has the kids fri evening until sunday.