After this period, they can continue driving in France with a valid UK licence, but it is advisable that they exchange their UK licence for a French one within two years of taking up residence in France. This can be done by applying in person to the prefecture in the department where they reside.
It is important to note that, for holders of a UK driving licence, only written exams are required to exchange it for a French licence. No driving tests are necessary, as the UK driving licence is recognised in France.
You can find more information about this on the French government website: https://www.service-public.fr/particuliers/vosdroits/F33160