Commissario
Wow you little snitch
He was wrong to consider going into work... but you could have said that upon picking him up instead of running to your boss, you massive jobsworth.
The op doesn't say when he was told - it could have been just as they were pulling up to the works entrance, or halfway there and too late to say "well I'll leave you here and you can call in sick".
The op may also have expected the guy to have been given an official warning and sent home which might have had much more impact that a quiet word from him, not to mention if someone is willing to do it once who is to say he isn't willing to do it when the op isn't giving him a lift? (for all we know the guy could have done it before without the op knowing and been warned previously).
We don't have much information, other than the op took one of only two options that he could have taken to protect himself, and the other workers from a self confessed potential danger.
I feel sorry for the guy who got the sack, but that is tempered by the idiocy of going to work where you know you'll be using heavy machinery when you've been drinking.