Tough situation. His boys are really young but this much I do know. Your husband should document everything. He should keep detailed information regarding how his ex-wife is behaving and influencing the children to their detriment. Email is his best friend. If she is crazy enough she will hang herself electronically. With enough ammunition against her she could loose custody and be forced into having supervised visitation.
In my husband's case, the ex-wife have telephonic visitation eventhough she lives in the state but the child is much older than 4.
He will need to go back to the courts. Courts aren't interested in he said she said, they are interested in facts. Keeping the facts straight via email would be very helpful in him keeping in touch with her for documentation purposes.
Also video and record phone conversations may also be helpful. Understand you do need to let her know that you are recording the phone conversation.
Keep all receipts and records of all monetary substance paid on behalf of the children. This will be important as well. You want to show you purchased clothes for them and sent them to her. Also with the medical anything you want to show evidence.
Their father must never give up on being a father to his boys no matter how hard and painful this process will be and can get. Eventually these boys will probably turn on their mother if her words prove to be lies in their eyes but they also will protect her because she is their mother.
You stay supportive and encouraging. My prayers are with you.