Probably nothing, because I would either be the one telling him the answer or be the one copying off him. Either way, I would say I'd just be an accomplice to the crime.
I generally ask stuff in the examination hall, or in the worst case peek answers off a colleagues answer sheet. But I am scared to death to use any other unfair means like cheat papers and stuff. I know I have no right to take the moral high ground, but if I see him doing that. I'd just warn him after the exam and mention it coyly without sounding too preachy.