If the custody orders require that your husband has visitation, and the custodial parent
violates the order, then (absent emergency circcumstances), that would constitute contempt of court and would permit your husband to ask the court to find contempt, obtain "make up" visitation, probably obtain a monetary sanction against the other parent (or, maybe some community service) and possibly even permit the court to reconsider custody in favor of your husband.
The problem with these situations is that it is very difficult to prove the frustration of custody/visitation, because there is frequently no one observing the situation except for the two parents, who can be used as a credible and objective witnesses. As a spouse, your testimony would be suspect. The police/sheriff are rarely willing to actually come to the custodial parent's home, and observe and report on the dispute, unless there is already some sort of breach of the peace or physical assault by a party on another.
So, this leaves the noncustodial parent with few options to prove the contempt and violation of the custody orders -- since proof must be "beyond all reasonable doubt."
Thus, if your husband really believes that the custodial parent will frustrate visitation during the winter holidays, then he may want to send a certified letter to the custodial parent, restating the express visitation orders that are potentially in dispute, stating that he understands from her oral statements that she intends to deny access to the child during the holiday season, and that unless he receives written confirmation that she intends to follow the visitation orders by ??/??/???? (give her 10 days), then he will assume that his understanding is correct, and that he will file a request with the court now, to clarify the holiday custody orders.
The point of the exercise is to put the other parent on notice, and then set up a court confrontation before
the holidays. Otherwise, it will be too late. Then, if despite all this, the custodial parent still violates the orders, then your husband needs to be prepared to prove it -- which could require hiring a private investigator to observe his attempts to exercise visitation.
I realize that all of this seems absurd, but I can tell you from experience that most of the time, a custodial parent who wants to frustrate custody/visitation, will succeed, because the noncustodial parent cannot obtain the required proof. Consequently, the only way to get a certain win, is to be highly proactive.
Hope this helps.