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Since they have equal custody then she can not take the child unless your husband agrees and she is willing to pay the costs of the child traveling to and from her home for the ordered custodial time he is entitled to. He can answer in a few different ways:
You can find a mediator HERE and could state in your answer that he wishes to use this service to explore their options on the matter if they can not come to an agreement on their own; for more help go HERE. If there is already a COURT ordered custody agreement, then the court will have to agree to any change in that agreement of they are in danger of custodial interference (kidnapping) laws if the order is not followed by both parties as well as it possibly backfiring on him later and her making false accusations to a court just to get custody.
So, basically he will simply state whether he agrees or objects to this notice and its contents and whether he wished to discuss it further in mediation or wait for a court to decide.