I wouldn't take what Aviva say as being common to all insurers in France. I left there two years ago so things might have changed but my insurers Assurance Moto, scooter... Assurance Mutuelle des Motards
were ok to have non-residents as named riders and so insured tous risques (fully comprehensive) and also to provide third party insurance for any qualified rider I chose to let on my bike.
As for your friend buying, registering, and leaving the bike with you, I would pay a visit to your local Prefecture and ask, these things seem to vary from department to department, but when I initially lived there I had no permanent address so registered my car at my parents who were able to provide me with the above mentioned justicatif de domicile.
Final thought, when you move to France you have X number of months to re-register any vehicles you import. As your friend isn't taking up residency perhaps he could store a UK registered bike there, with UK insurance. The only problem would be UK Road Fund Licence and MOT, but a once yearly trip back to the UK on it would solve that, although if he bought new and sold before 3 years he wouldn't have to worry about MOT, and so would only need to pay a visit to a UK Post Office with his documents. Not ideal, and probably more expensive than doing it all in France but it is a possibility.
Have a Google for the Anglo-Info site relevant to your region, they have fora where you can probably find someone who has up to date experience and can help. You'll possibly get nine different answers, like any forum there'll be those who actually know and then those who like to think they know.
Happiness has 125 cc