Couple of things on this thread.
Reg keeper and owner are two seperate things, so one can be reported seperate to the other for motoring offences. Rob114 is quite correct as regards registered keeper in this case (but there are others where it is NOT the case)
See previous threads regarding Driving Other Cars insurance question (DOC extension) for fuller ins and outs.
In the briefest of circumstances as originally posted in this thread, the owner did not view any insurance certificate claimed to be in force by the person to whom the car was lent. I would suggest that this was an unfortunate oversight which may well have an equally unfortunate ending for the car owner in court.
Yes, 143(3) allows an exemption but this only applies to employee/employer. The car owner (employer) is still liable for insurance requirements and the car driver, employee, would be a witness.
In original case, the car owner causes or permits the no insurance offence by permitting the driver to use it on a public road. If he/she had said no, then car would no have been used and no offence committed.
As others have said, unfortunately ignorance of the law is no excuse and I would not be surprised if the 'chat' turns into being reported for a 143 as well.
The alternative which prevents the owner being reported for any 143 offence is if the car was taken without the owners consent. The driver would be additionally charged with a suitable offence (TWOC in England?? - RTA 1988 sect 178 up here) but this avenue is fraught with difficulty and danger if not strictly true..........