Eric,
With respect, kit cars are
not illegal here. Identity transfer, however,
is illegal.
If you have a car first registered over 25 years ago (classic status) then import & registration with a French plate is relatively painless.
If not, then you need to visit the DRIRE for type approval for "low volume production". It's not as simple as SVA (37 different tests at €100 each last time I looked into it). It can be very long and very expensive, but not impossible.
Here's a road legal 'Busa powered XTR2:
http://www.autojournal.fr/video-auto...net-10887.html
One other alternative - I think both Caterham & Westfield sell their 7 replica here (turnkey only, with type approval). You friend could speak to them for assistance.