You can do a hobby for any reason. It doesnt have to be for yourself. He could enjoy programming but also enjoy contributing to the wider open source community. So it makes sense to feel bad when he sees another project popup and invalidate his work. If it was another person who'd worked hard and thought about the project he could at least take solace in chatting about their solutions but with AI its nothing. It just solves the problem and there is little to be gained from it.