Borrowing something, and it breaks, who is responsible to fix it?

Best policy, don’t borrow.
My husband borrowed his brothers backhoe. Used it for the day. He was going to take it back the next morning. However, it had a flat tire in the morning. This was an old crack, almost bald tire. Took it to the tire repair, couldn’t be fixed. My husband replaced the tire with a good used tire. Had it put on and then returned the backhoe. Very expensive lesson.