Not all that much THIS time, if he keeps it up, it'll get worse.
If he pleads guilty, he'll have a theft conviction on his criminal record.
It is a federal conviction, and he might find job opportunities limited for a very long time.
He could lose his Exchange privileges if he persists in this foolish behavior, but the FIRST time that usually won't happen.
He's likely to get a lecture from the magistrate, be required to pay some fines and court costs (that is in addition to what he paid to AAFES), required to take a class or two on ethics/honesty, do 40-80 hours of community service, some people have to take drug tests, most people get a probation officer who'll determine when he reports and where, the place where he lives and works will be subject to random searches at all hours of the 24 hour, 7 day a week clock, and he might have to undergo a psych eval.
Each case is treated differently, so as the Mickey Mouse Club would say on Wednesdays, "anything can happen day".