When trying to convince a friend not to drive after drinking what should you try?

You shouldn't have to rationalise with a drunk mate who wants to drive, there shouldn't even be an argument in which you have to convince him not to. If this person is over the limit/not in the right state to drive then you shouldn't let them. Take their keys if you have to. They should thank you in the morning.

You very well could be saving lives by not letting them get in that car, so do anything you can.