When they first arrive and in advance of getting accepted into the French healthcare system, they can go to any doctor they like. In France you don’t need to be registered with a doctor to make an appointment to see him/her. They can visit any doctor they like (and can get an appointment with), and pay the consultation fee (20-odd euros, I forget how much exactly) and the cost of any prescriptions. If they keep the paperwork they may be able to claim part of it back either via their EHIC if this is a holiday home and they will be in France as visitors, or from CPAM if they’re making a permanent move.
If it’s a permanent move then once they’re registered with CPAM it’s a different situation, but you can’t apply to join PUMA (the name of the French healthcare system) until you’ve been here for at least 3 months, and it can take a while for the paperwork to be checked and processed, so as Stella says they would be well advised to plan carefully for their medical needs in the first few months. Also, to speed up the registration process as much as possible, they should check what documents they will need to go with their application - birth certificates, marriage certificate if applicable, proof of income etc - and start getting their dossiers ready before they arrive. I think sometimes French translations of some things are required.