I might be wrong but I believe the scenario you are portraying is not accurate.
in that case, the French student could get his license without any issue from the French DGAC.
The only thing he would need is an EASA Class 1 from France (or from any EASA member state but then to apply for his license in that country). Licencies will be issued by the country where you had your medical.
I believe a better analogy would be if a student goes to the US an completes all his training there and then goes to the French (or UK) CAA to obtain their licence...
Going back to your example, what happens is that the Uk CAA is the one that decided to move out from EASA and therefore cannot issue an EASA licence anymore. If the French student applies for it from France, he can then go to the UK CAA and convert it. I believe the UK CAA will accept EASA licenses for up to 2 years from Brexit.
A similar case happens now with some gulf countries that replicated EASA regulation and study requirements but are not EASA member states... so to convert a licence from those countries requires the same as if it were done in the US (or any other ICAO licence)
It is just one example of the implications from Brexit. A lose-lose gane for everyone in the European aviation industry, I am afraid
don’t see much to challenge in court... but that is only my humble opinion