Its OK for him to ask unless the tenant is a child, and there is proof he wanted more than just to give the kid the thrill of riding a lawn mower........if you are an adult, what is the big deal? If he is trying to get a thrill, then tell him it is inappropriate to do that and not do it again. But there is little else you can do. And, check your lease, you may have to move as soon as its over.