I m not a legal advisor or even an expert in law,
But i would state that if he had driven this car from france to england for the duration tax would not be needed. However as the vehicle is registered in England and will remain here for a period exceeding the " travel home from purchase " then it would be illegal to drive the vehicle or even park it on the highway without the current tax disc being displayed , with the new impounding rights of the police being in action this could be an extremely expensive action to remove the tax disc for this period.
I would suggest that you sell the new owner the tax which would allow him/ her to drive the vehcle for the duration and allow them to send it back for the refund.
foot note
Your refund for road tax is payable by the complete month only no refund is given for any part of a month, so with this in mind and at date of posting this being the first day of a new month , no loss will be made for leaving the tax on the vehicle and sending it in " within good time " for refund , upto the end of the month.