Well I'd expect him to end up in removal proceedings.

There's two ways to fight this, first method is to fight the initial charge if he's found not guilty, the charge is dismissed, or is plead down to something that's not a felony or CIMT, he'll be fine, there's nothing that'd make him inadmissible and the proceedings can be terminated since the original NTA was defective (non-existent charge). Basically try to either get it dismissed, if that fails try to plead it down, if that fails fight it and go to trial.
Second way is if he'd get convincted. Since he was detained on entry he's an "Applicant for admission", meaning that you can seek a CIMT waiver using form I-601 before the immigration court (assuming that he has a qualified relative i.e. a USC/LPR Spouse/Parent(s)), if approved he'll get his green card back. Back when I worked at a law office we got a guy who had two counts of third degree assault off the hook, he was detained on entry so we were able to file I-601 for him using his LPR parents as qualifying relatives, it got approved, and he was released and got his green card back.