Heh - the debate only proves my point.
How does magic work in your world? Or, to paraphrase, what's the theory of magic in your world?
Two cases can be made for your specific instance...
When magic is cast at a target, that target can 'sense' the magical effect occurring. At this point, instincts can kick in - screaming "NO" and/or be consciously overridden.
-or-
Saves vs magical spells are a conscious thought that once overridden, can't be changed.
As a third 'theory' of magic, try this...
I, the fighter, will allow a 'zone of truth' to be cast on me.
The wizard casts Charm Person on me.
This I resist.
Do I, as a fighter, need to know what spell is being cast in order to choose to resist or not? If so, how do I, as a fighter (in game, not metagame) know whether to use Fort, Will, or Reflex?
The point of a trap being in a dangerous area is irrelevant. Traps can be anywhere, saves vs magic can happen anytime in DnD. Living life as a fighter in DnD is much more dangerous than driving your car... and think how dangerous that is.
I disagree with the latter as a player and as a GM. It opens the field to far too many dangers. Just because I agree to a spell doesn't mean I'm not going to resist it if my instincts scream at me loudly enough.
Of course, in this specific instance, it boils down to how much you trust that wizard? From a metagame - the wizard can't know if he succeeded or failed until he tests the spell. The fighter, if there isn't a STRONG level of trust, might well have resisted in a general fashion.
However, don't get bogged down in the specific situation - think of the generic one.
You should always be allowed a save vs any spell that allows it - unless you choose to not take that save.
In this case - if you role played the whole thing well enough, the fighter would never have clued in the first place, and thus the whole question becomes academic.
I know where I stand - I'd've rolled the fighters save for him without telling him anything about it. I'd trust the player to role play through it and not metagame

.