In general terms, you should treat the skipper as a guest.
If you are hiring a professional skipper to join your charter, they are onboard to deal with the sailing and safety of the yacht. So when you're not sailing, they are a guest of yours. So when provisioning, make sure you buy enough food for all of the crew - including the skipper! If you are eating out, you can invite them to dine with you, alternatively, you can give them some extra money to find their own food.
When chartering a yacht with a crew, this cost will be included in the charter fee, and the crew will take care of it themselves.