I would think a TA would fall under school official who needs access to do their job, which does not violate FAFSA. A TA, paid or not, is functioning as an official from the school, just like a teacher.
As long as you aren’t grading your classmates and are functioning as a TA, I don’t see any problem with that.
OP, I think it would be a great opportunity for you, especially since he will be writing a rec for you. You find yourself TA’ing in college somewhere down the line.