if there is a serious injury to your neighbor you would not be able to certified the maintenance of the equipment nor the training to the user. My thought is that the risks outweigh the benefit especially if the son is minor.
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Generally speaking, you're only going to be responsible for something stemming from negligence, for example, if you fail to warn him of a hole in yard that he breaks his ankle in, or if your equipment is poorly maintained and somehow injures him in a way that can be traced to your own fault. I know of a person who fractured his finger trying to start a pull-cord mower, raising question of a design defect, which would not be your fault. If the kid stuck his hand under the blade guard and cut off a finger, that's his own fault. You might want to make sure these folks have medical insurance in case something does happen, then maybe make a simple agreement with them that you will pay $x for his services and he's understood to be an independent contractor and that you won't be responsible for injuries.