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When they were living here under their EU rights, they didn't need to do any of that stuff, so you cannot blame them for not doing so.
I’d need to look up the relevant French law but when I lived in the UK the same rules existed as do here ( in that you need to change your foreign license for a local one). Having a license from
elsewhere in the EU didn’t exempt you from that. The fact that the rule was rarely, if ever, enforced doesn’t mean it doesn’t exist.
When they were living here under their EU rights, they didn't need to do any of that stuff, so you cannot blame them for not doing so. In fact the French were refusing applications after the brexit vote but before actual Brexit because their British licences were still valid. And then they wonder why they got overwhelmed afterwards! It's a shambles by a typically inept French bureaucracy and an incompetent British government.