A friend of mine is getting "unfair" treatment while deployed. What options does he have?

The Army isn't "fair" it's a meritocracy. One doesn't "manage to get adverse action". A person doesn't accidentally step in a mud puddle and get flagged.
Your friend got into trouble. If he was barred, and the PSGs recommendation to the Commander is to leave that bar in place, then your friend needs to rehabilitate in order to overcome that bar. For adverse action that usually means showing sustained and continuous leadership potential - not simply "not getting in trouble for a while".
To quote someone else, "Retention, like spandex, is a privilege - not a right".

A bar is not like a flag, where the reason for the flag goes away, so must the flag (ABCP, APFT, UCMJ finishing etc.) A bar is based on the whole Soldier concept. The PSG doesn't have to fish for a reason for the bar not to be lifted, your friend needs a positive enough reason for his CO and BC to change their mind. If your friend got UCMJ (you said he had extra duty) AND he failed an APFT, it sounds like his hole has been dug. I'm not seeing the "unfair" part. His best bet is to sincerely Soldier up and do his best to kick ass in everything he does to the point where the leadership wants to keep him on their team. Crying "not fair" or looking for a legal loophole in all this shows a lack of introspection, it might be him, not them, and he might need to fix himself, there is no shortcut to staying in. Times are tough, I went from hoping to make SGM to being glad I didn't get QSP'd. Here are some excerpts for you and your friend to consider from the applicable reg.

AR 601-280
3–7. Commander’s evaluation of soldier for continued service
a. Commanders should evaluate all potential reenlistees under the “whole person” concept. Those soldiers who are not considered suited for future military service should be considered for immediate administrative separation or initiation of a bar to reenlistment under the provisions of chapter 8 of this regulation.b. Factors considered under the “whole person” concept are listed below. This is a commander’s guideline to assist in evaluating the potential reenlistee.
(1) Recent nonjudicial punishment.
(2) Repetitive nonjudicial punishment.
(3) Low aptitude area scores.
(4) Low education achievement in combination with a pattern of disciplinary incidents.
(5) Low evaluation reports.
(6) Slow rank progression resulting from a pattern of marginal conduct or performance.
(7) Potential for further service.
(8) A combination of any or all of the above factors.

Approved Bars to Reenlistment will be reviewed by the proper unit commander at least each
3 months after the date of approval, and 30 days before the soldier’s scheduled departure from the unit or separation from the service (see DA Pam 600-8).
(1) If, upon review, the commander feels the Bar to Reenlistment should remain in effect, he will notify the custodian of the soldier’s personnel records who will enter the remark below on the soldier’s DA Form 2-1: “Bar to Reenlistment reviewed; not recommended for removal, (date)”

SSG (Join to see) Not a lot to go on here. That said, there are at least 3 sides to every story. (1) The person's story. (2) The command or management's story. (3) And, of course the actual factual story. It sounds like your friend needs to find a new line of work. Being marginal is not in line with be a successful Soldier. This would be a situation where I would suggest the aggrieved looks in the mirror first.

But, if some is being treated unfairly, and they can't settle it via the chain of command, the local IG is a option to see what his/her options.