Digital Nomads

Sorry if this question has been asked. I’m just seeing a lot of different answers and not sure what is correct. I am working with a client who is going to purchase business development training. He has a tourist visa in France right now. He is worried about building a business because he’s not allowed to work. But isn’t he allowed to run a business out of the US while he is over there?

EDIT: I forgot to mention this is a completely online business. He would not be marketing to people in France.