I know guys who do this for $$ or for close friends knowing that they'll get something in return (help on a startup idea?) but teaching for free is odd -- tutors in other fields get paid, why should comp sci keep racing to the bottom?
I would be much more comfortable with "Code my startup project: I retain property rights, you get to learn". It's a much more even agreement. Not super exploitative given that the tutor probably could code everything they are teaching, but they let the student do it at a slower pace in exchange for some oversight (hopefully a lower time investment for the senior than coding the project themselves).