Well as other posts have said, could you live with yourself if something happened to him, I know I couldn't.
If you get into trouble you have no help, seeing he is not trained you have to assume you are diving alone, are you good enough to dive alone, or rescue someone?
A friend of mine saw an accident at a NSW pier.
An instructor and his "friend", wend diving off the pier, one scuba set between them. My friend, an instructor too, ended up pulling the drowning instructor out and while performing CPR saw him die. Utter stupidity to have one scuba set between any two divers let alone a novice and a certified diver. The instructor paid for his stupidity, but it could have been the novice or both.
Would you be legally liable, most probably in some sense so exposure to being sued or charged under some law.
Personally I would not, I care too much about my family and also would not expose myself to legal action.