It seems that this friend of yours needs to find an attorney that is well versed in this sort of thing.
It is going to cost a few bucks, but to regain or retain ones 2A rights is well worth it IMO.
If he is flagged a s a felon, I am guessing the gun he got caught carrying was not purchased from an FFL dealer. And why didn't the arrest turn into "Possession by a felon" charge too?
Something is not quite correct here.