In reading the OP, and what the OP later posted the player in question doesn't seem to care about his character so much, as just playing his alignment as a chaotic neutral. In my mind a CN character might just let the wizard have the magic item as getting it himself. He chose the item to remain with himself at that moment, but could have easily gone the other way, if his mood was different. If a charm spell coaxed him to let the wizard have the item, he might not know that it was not his decision to do so.
If on the other hand it was completely out of character for him to give up the item, then sure, there's a chance that he might suspect magic was used on him to force the decision. If so, he'd recognize that something like a charm spell was used on him.
I just don't think it was obvious, nor out of character for a CN - which could do anything (or at least that's what this player thinks a CN character might do). If the character thought he gain some advantage by letting the wizard take the item instead, he might just do that, and that too would be an appropriately CN tack to take.
This thinking wouldn't apply to every player/PC, but it seems it might be very appropriate for the one in question.
With the above understanding (as I see it), the fighter would have little idea unless somebody told him he was charmed, that anything untoward happened to him.
Then all of my previous posts are accurate.
Personally, I think using a charm spell on one of your own members is a dirty thing to do. As a player, I wouldn't play with a group that would take this tact. But then as a player, I wouldn't deliberately screw over another party member out of a magic item more appropriate for someone else in the party.
Overall, the party and this particular player are both dysfunctional.
But my reasoning still stands.
Edit: (citing rules from the Pathfinder Core)
Charm Person
This charm makes a humanoid creature regard you as its trusted friend and ally (treat the target's attitude as friendly). If the creature is currently being threatened or attacked by you or your allies, however, it receives a +5 bonus on its saving throw.
The spell does not enable you to control the charmed person as if it were an automaton, but it perceives your words and actions in the most favorable way. You can try to give the subject orders, but you must win an opposed Charisma check to convince it to do anything it wouldn't ordinarily do. (Retries are not allowed.) An affected creature never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing. Any act by you or your apparent allies that threatens the charmed person breaks the spell. You must speak the person's language to communicate your commands, or else be good at pantomiming.
(me now) Since the Charmed Person in question is already an ally, it would be even less a problem to convince them to do a party friendly act. If the action desired, is perceived to be in opposition to the charmed person's goals, they would receive an opposed check (don't know if that was done in this particular instance). If they fail their opposed check, however, nothing in the spell description suggests that the player or other Charmed Person would know he was charmed after-the-fact.
