well if this person is wise, he will listen when you tell him that his prophet (peace be upon him said) the following:
"whom ever cheats is not from among us (the believers)".
but I doubt he'll care, he may even laugh at you and say "when did you become an extremist" this is the case with alot of the youth, try and drum into him
"bro your prophet said this, don't you believe in him? he'll say he does, then say
"well follow his teachings then and show your belief in your actions by not cheating and obeying him"
not "I believe and do whatever I want" belief is shown in actions.