According to a very strict interpretation (my interpretation, in this case, obviously) of the RAW, sure, the warrior can choose to save against the Wound spell. Here's why:
Voluntarily Giving up a Saving Throw: A creature can voluntarily
forego a saving throw and willingly accept a spell’s result.
– Pathfinder, Saving Throw
Emphasis mine. The result of the spell is that "you lose X hit points", and the question is: "do you willingly accept that?"
Of course, it could be argued that by "result" the RAW refers to the "result as expected by the character (not the player)"... but that's already adding to the text and interpreting it.
Casting goes like this, in my understanding: Caster tries to, and if nothing interferes, casts the spell, spell takes effect, that is, its (intended) result becomes known, saves and whatnot are applied to said result, result is applied.
Even Will saves don't necessarily reflect conscious decisions, choices. They are pretty instantaneous - and the nanosecond the character realizes the incoming energy is cold instead of warm (or something along these lines), her Will, her self defense comes into play - unless her Player opts for giving it up and taking the damage because said Player feels the character trusted the spell's source truly, madly deeply. :)
That's my take. The warrior gets to save.