if they are working for a company based in France, then they would have to continue paying tax there, right? They only need to pay tax in Thailand if they work with a company or business in Thailand. Unless France has double tax agreement with Thailand just like Australia, then they can choose to be taxed either in their home country or in Thailand. Did I get the information right?
You might need to check this properly though. My understanding is that DTV is a tourist visa and you cannot work for a company registered in Thailand, although the company may have branches in different countries. The DTV is for foreigners who work for an overseas country which allow them to work remotely or as digital nomads. Your situation doesn't sound like you're on a workcation, you literally are working in Thailand and you may need to get a proper work visa instead. Worth clarifying this with a visa expert just so that you don't get into any trouble with the immi krab.