SGT Abusing power

I doubt that you have the story right, as 1sg's don't directly supervise soldiers. Most likely it is a platoon Sargent that is the problem. In any case, if a 1sg is going after a soldier there are in place the platoon Sargent (Sargent or staff Sargent), the platoon leader (lieutenant), probably some Warrant Officers, the company XO, and the company commander(Captain) whom all have a responsibility to fix the problem. If that does not work the soldier should speak with the Chaplin. If that does not work the soldier should see if the Sargent Major has a open door policy. If that does not work them the soldier should file a complaint with the IG (inspector General).

I agree with Hawkeye about everything he said generally. In my experience, even though a couple of times I've found myself at the mercy of an NCO who obviously disliked me and let it show in unequal treatment, usually you can find some avenue through which to have concerns heard about an unfair leader. If the person in question is in one of those units where abuse of junior enlisted is sort of part of the culture, it could be that there awaits a long road ahead of him.