If he doesn't appeal while on active he is giving up opportunities to have the wrong fixed. If he doesn't use his levels of appeal, and he in fact needs them, he is going to have a harder time appealing after he gets out. It's in his best interest to give the government an opportunity to fix the wrong at every level. It's short term pain for a potential long term gain. If he walks now and doesn't appeal when he should, he should probably come to terms with the fact that he will back himself into a corner and successful appeal later will be extremely difficult.
Imagine he has two levels of appeal available to him and he chooses to take severance over any appeal. Federal Court and BCMR are going to want to know why he didn't appeal then. It's kind of like saying, "I agree now, but then later I don't because I missed out on a lot of benefits."
Do what you can to convince him to fight it now rather than later. It will be time consuming and possibly more expensive to fight it later.