When he's driving it, he's driving it..
Unless he's still making his breakfast whilst driving.
In which case I think that comes under 'driving without due care and attention'
As an offence - and not OK ever, wherever you happen to sleep.
I don't think having a whole bunch of old clothes, and some food in your boot invalidates your insurance.
... If it does, I'm in big trouble..
The 'where you keep your car/ registered address' for insurance thing, is a slightly different matter.
But I don't suppose your mate is going to make a claim for his car being nicked off of the driveway of his poste restanre address, if he was nowhere near there at the time..