If she is being charged with a possession offense it seems that should eliminate his possession offense.
In addition to what Zigner pointed out, if she was charged with possession because she too was a felon then that would open the door to the prosecution asserting that the OP was also in possession of it under the accountability doctrine that Illinois courts recognize, as described in an earlier reply. Thus, as I indicated earlier it matters a great deal what the charge against the wife was.
Bottom line, while the wife's possession of the weapon while standing next to the OP would not by itself be enough to make the OP guilty of possession of a firearm by felon, with additional facts the state may be able to make that possession case against the OP. I see two possibilities from what we know so far: (1) the state argues that the OP directly had possession of the weapon in his waistband based on the witness testimony of the other combatant in that parking lot brawl or (2) the wife is charged with possession as a felon and, since she was present when the fight broke out, the state asserts she was a participant in a crime related to the fight (e.g. battery) and thus the OP had possession under the accountability doctrine. The former seems more likely to me than the latter, but we need more information to sort out what the state is going for here. It should all be spelled out in the complaint/indictment the state filed in court.