The bill is yours.
As it is he is already into this deal for the cost of dropping off the machine and now the downtime will cost him because you broke it.
Would it have broke on the job with him running it? Maybe, but the hoe came to you in assumed working condition and was in working condition up until the last day you used it. He wasnt running it, you were.
If it was mine and you were my "friend" and you tried to stick me with a bill after breaking my machine, we would no longer be friends and you would be getting a bill for hoe rental, delivery, and fuel. You can bet that it would be at least as much if not more than the repair.
Machines break, even if it was through no misuse. I bartered a york rake for my yard. Even though goods and services were exchanged, when I broke a tine, the first thing I did was call the owner and ask him how much was a new tine going to cost me?