Buyer admitted to modifying item before returning

I sold a Christmas tree a few days ago and the buyer opened a return saying that it didn’t work. All of the lights were working when I sent I to them. But he claims that there was a section that was burned out. So he tried replacing the bulbs with ones that he had and also replaced the fuses. The tree didn’t come with replacement bulbs/fuses. I feel as though he is going to return a used tree and keep the new one I sent, or he was making modifications and it didn’t work. Are buyers allowed to modify items before returning them? Whenever I receive something that isn’t working I simply return it to the buyer in the same condition that I received it in.