That is weird - the C one octave above middle C is not very high for a trumpet. They can usually go at least one octave above that. I wonder why your trumpet player friend said that - maybe he was talking about high school 3rd Trumpet players which aren't very reliable in a range higher than that? I can't really think of another reason.
Are you sure you don't mean to be asking about the written C6 - D6 for Bb Trumpets? (which would be a concert Bb5 - C6) 1st Trumpets even in high school can usually be expected to hit a C6 but not too much higher than that.