[antonye]

I think the answer is about being drunk in charge of the *keys* to the vehicle rather than the vehicle itself, if you see what I mean.
If you have the capability to start/drive the car (ie the keys) then you are "in charge" of that vehicle and can be nicked for it.
Personally, I would explain it from the point of view of the morning after; he's very likely to still be over the limit even if he feels fine, so could still get stopped and fined/banned.
Have a Google as I'm sure there will be real examples.