He's ok doing all that shady stuff but too much of a ***** to hand prop it?
I can always trust DW to say the things I bite my tongue on.
As far as the hot start goes, it'll cool off. And maybe he would too after he thought about it for a couple of hours.
As far as offering to hold the brakes, that is the last thing the OP should do. The OP works for the FBO. If he assists the guy with hot fueling, and something goes wrong, the OP is now just as liable as the pilot.
Worrying about what the lawyers will do after the fact is never a good reason to not do the right thing. If the guy is dead set on doing something risky and you don't tell him he's crazy but you'll at least lend a hand to make it marginally safer, you're an awful human being.
Hell if you're worried about the lawsuit just whip out your camera phone and videotape you telling him he's an idiot but you're not going to let an airplane sit running on your ramp without someone at the controls.
Or, if you really want to do the right thing, reach inside and shut it off. Now he has a new perspective and an all new chance to make the right decision. What's he going to do, sue you for shutting off his busted starter airplane? Let him yell and whine. He'll get over it.
Nobody's going to take his side in a "get-there-itis" attitude that was that bad.
Standing and staring: Worst possible option.
Not shutting off pump: Also bad.
Offering to hold brakes: Better. Still wrong but better.
Shutting off his unattended aircraft: Best decision you'd have made all day.
A jury would ask why you didn't do the last one. Especially once they heard he walked off to take a leak.