We've done Hertfordshire to Bourg a couple of times.
Did it all in one go on the return leg once - it's doable but it's a slog and as said above, the driver will be knackered and need recovery time.
I'm sure your mate will be aware, but don't forget the VE103 - it proves you have the vehicle owner's (lease company) permission to take the car overseas. If you're stopped without it, the French police can impound the car (never heard of this happening, but it could in theory). It's nothing to do with insurance though.