Well, the issue is, he asked you to come over and do work unpaid, as a favor. There is not fault. And as a favor back to you, he offers to pay for repairs, because he may feel guilty having you come to his property and do work for him and your equipment broke down. Thats how I see it. Well, I see it as 50/50 if I am unpaid and the price of repairs is high. If it was something like a bolt or hose, then yeah, I would totally pay for it all 100%.
There is a bigger picture here than I think many people are not seeing. The relationship you have with your friends and neighbors, that span decades. Favors exchanged, beers bought for one another, your neighbor sleeping on your couch because his wife kicked him out type of thing. It's not just some random dude you met once that asked you to work for him unpaid. If that were the case, this wouldn't be an issue (it's not really an issue but more of a thought exercise), it would be contracted over a handshake and an agreed upon amount for the task. It's like your best friend calls you because he needs your help moving a couch. Are you going to charge him? I hope not!! But what happens if you throw out your back helping him? Hopefully you have insurance or its not that bad to see a doctor, but if you had to pay out of your pocket, for helping your best friend with a favor that is different. If I were the friend, I'd feel so guilty that I would insist on paying!
It kinda seems like people don't do favors for one another, least not without a contract or prearranged negotiations on what happens if a meteor falls on your equipment. I actually do contract work with my tractor for others. They are NOT friends, but more like acquaintances. They call me and ask to use me and my tractor for whatever and to 'name my price.' I throw out a fair and agreed upon number and time frame then get to work. That situation is different. When someone who routinely does favors for me ask that I do a favor for him, there is no negotiation on price. You just get to work. And if something breaks, he helps fix it, especially if he is right there working along side me. I actually felt kinda guilty because he was doing manual labor and I was driving the tractor!
It is not so cut and dried. Granted, I do believe that ultimately I am responsible for my tractor. And that is why we agreed to each pay half. He felt guilty, but its my tractor that I was operating. We went back and fourth for a while, while drinking a delicious beer, figuring out what went wrong and best way to fix it.
I am fully prepared to pay for any damages I cause to my equipment wherever it happens. But sometimes, maybe you should look at the total picture, take your friend up on his offer to pay, and get the job done.