I'd like to know how many tach hours the guy put on the airplane, if it
was topped off when he took it, and if he refueled it.

My own fuel management routine is simple: take off on the tank I did
the run-up on, burn half of that away (in a M20J there are 33 gallons
on each side), switch to the other tank and burn most of that away,
come back to the starting tank and land for fuel. I can lean to get
less than 10 gal an hour, so the airplane has long legs. I never have
landed with less than one tank half full, and that includes very long
cross country flights like CA to MA. Unless the PIC in the OP had a
really solid excuse, I wouldn't let him fly me unless I managed the
fuel.

Here's the thing. There's probably an increased chance of this guy
being PIC and making a bad flight decision. If it happens, the hand
wringers who are concerned about damaging his career will feel badly
for their inaction. Send a note to the chief pilot of his airlane
outlining the facts as you understand them. You'll have at least
offered a decision maker a head's up. The chief pilot may choose to
investigate or not, but that is not your problem.