What he said was hardly the crime of the century. It's just a small matter of convention. I thought the answer would be obvious, have a quiet word in his ear pointing out the mistake.
I wouldn't talk to him within earshot of a student and I wouldn't be having a go at him, so I fail to see why it is necessary to check what everyone else would do before acting.
The only options are:
1) Do nothing, just chuckle internally everytime you see him.
2) March into the briefing room and castigate him infront of his student.
3) Talk to him privately.
Or I suppose there is a 4) Start a thread on a well known flying Bulletin Board and see if he reads it and recognises what he said.
As already mentioned, it's not a conventional way of talking about distance and speed, but it's hardly completely incorrect either.