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I assume a CFI needs to be current in cat/ class takeoffs and landings to teach a student, but what about a non-night current cfi teaching a commercial guy at night?
[/ QUOTE ]If the "student" may and does take on the role of pilot in command for the flight, the CFI doesn't require =any= currency. Heck, he doesn't even need a medical certificate.
So, in your example, if the commercial student is current (including night currency), has the qualifications to act as PIC in the airplane, and does take on the role of PIC, the CFI does not have to be current in anything, let alone night takeoffs and landings.