If said driver is arrested for DUI (which is sort of a bad example since one can usually see when someone is real drunk, apart from seing who is a licence holder and who isnt) will the landlord get a ticket? Or should rather all other drivers be required to do an alcohol test before driving?
No there is very unlikely to be any criminal charges for the third party - but everyone has to base their actions on their own ethics / morals.
If you (as an FBO - or anyone else who may be aware) see a single pilot operating a multi-pilot type, then maybe you would consider what the consequences of just a pilot flying your family or taking their jet over the house of your family.
If I saw it and was certain this was happening then I would certainly make it known to the tower / authorities to prevent them getting airborne.
Donnu how that is handled in the UK, but if I lend my car I have to check if the person has a valid licence. If an accident happens and the drivers got no licence, sh.it hits the fan for me as an owner...
Having passed through 4 UK commercial operators, I had ALL my documents scrutinised before I was able to operate.