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That's a tough call. If his battalion commander specifically stated that he does NOT have to report back, then he should stay on top of this situation very carefully and be in constant contact with that commander.
If the battalion commander has not said that though, he needs to report back or face being AWOL. The fact that his honorable discharge was set to take effect is not a legally relevant factor here at all. That is tentative paperwork that can be changed at any point prior to his actual ETS date. Terminal leave is still leave, so it means he is still subject to military jurisdiction. If they are stopping his leave based on an investigation, they can do that.
So, everything here depends on the exact wording of his battalion commander's statement.
So, just to be as clear as possible, unless the battalion commander told him to ignore the order, it is still a legal order.
He would make the situation worse by not following it. Nothing in your facts here make it an illegal order. The only reason he'd have to be able to legally ignore it is if the battalion commander expressly stated that he should wait until the matter had been explored.