I am very sorry for this but the behaviour of your DiL is quite normal. Many parents, not just mothers, try to cut their chldren off from their partner and thir family by accusations of abuse and violence.
All your son can do is persist in constantly drawing her behaviour before the court. He should also continue to send cards and presents to the children, making sure he photographs what he has sent, and has a certificate of posting, so that he can show that he has continued attempts to keep in contact with his children, similalry with emails, texts or other communication.
He, himself should never say anything against his wife or lose hos temper. Ahead of him is a long war of attrition. Keep records of everything she does or says.