I hope you won't be loaning him anything again.
Whats the old expression of things not to lend others? Chainsaws, sleeping bags and wives? Might have to add mowers to it.
Especially mowers with unforgiving bar blades!!
I used to loan my rather heavy handed father my Victa Powertorque Mustang and while I knew he wouldn't straight fuel it (he taught me mixing fuel), blades wouldn't last a day with him, which is why I would always loan him that one and only change them when they resembled popsicle sticks!
That mower still runs today, years later. With a bar blade set up, it would have been all over before the fat lady sings!