Most folks today are not the same as yesteryear when you loaned equipment, vehicle, etc. you would not only get it back asap full of gas clean and running. I've encountered the same issue in times past and finally decided to say no. It's hard being a good Samaritin these days. If someone asks to borrow my tiller I either tell them I'll do it for you or where to go rent one. I believe it's only fair, fitting and proper, depending on your labor and parts, to bill the man for his abusing your equipment as well as his incompetence.