He's lucky it was shallow enough to save the engine, but like others have said, I'd be concerned about the fluids.
As for ice thickness, it can vary so much in a short distance. I've been ice fishing for well over 50 years. I never trust it.
Rule of thumb is 4" of ice for a person to walk on. 5-6" for snowmobile or ATV, and at least 10 for car or small truck. 12+ for larger trucks.
With that said, I've been on very solid 3" of ice and very weak 12" of ice. Gotta check every time.
There are some folks that shovel off a skating rink for hockey on a local lake. Sometimes a small tractor with plow is used. But they use an ice auger and check the thickness in the area before they take the tractor out.
I have 1" alternating red and blue stripes on the shaft of my auger above the flighting. Once I drill a hole, I just pull the auger up until the flighting hits the bottom of the hole, put my thumb on the shaft at the top of the hole, remove the auger and count the stripes. Pretty simple and only takes a minute.