How to handle someone propositioning me to collude/cheat with them on an assignment?
Putting such a request in a written, verifiable manner as this student has done is incredibly dumb, and frankly merits whatever punishment is associated with this. 

Although in principle you could simply ignore the request, I think this is one of those cases where you're better off reporting it. Otherwise, there's still the possibility that "John" could bring you down with him (he wrote you a note, after all!). So I would write to him declining the offer, and then report it.