If I can turn his skills into money, depends less on the engine, than rather on other terms.
Imho, this is not really a question of Unreal Engine vs Unity.
So, I suggest, rather go first with the one you are more comfortable and second, depending on the way you want to make money with it, Ie. if you want to create and sell plugins, check which market allows it for you better, after doing your research. Then again, for some things, it’s possible to provide your skills to both engines, as many do in some areas.