That sounds like an unlawful order and not something "his sergeant" can order anyway.
He should file a complaint with the Army Inspector General, AFTER he first brings it up with his First Sergeant and Commander. He should use the chain of command initially. As part of working his way up the chain he should ask for a written counseling statement as to the supposed reasons for this order.
If he gets no response from the command, then it's time to go to the IG.
He should also go to the Base legal assistance office. They have military and civilian lawyers there who will be able to assist him.
That sort of instruction doesn't make a lot of sense. Your husband may want to talk to a local JAG officer to seek an explanation of the prohibition.
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Mr. Harkess is correct. The military normally triads to keep couples that are in the military with small children intact. Your husband has free legal counsel with the JAG office on his base and he needs to use the JAG officer.
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Your husband has access to free legal counsel in our military (please thank him for his service). He should sit down with that counsel, found in the JAG office on his post, and have a talk.
Good luck to each of you!
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.