I guess he's thinking because the 4th of C major is an F natural, which could clash with the F# of the key signature. If he's playing a Cmaj7 chord that's not the case though. I can't see it making a big difference to the tonality just tell him its a passing note if he tries to argue.
Remember, theory is descriptive, not prescriptive. Its a way of describing what you're doing, not restricting what you can do. Ultimately, if it sounds good, it is good!