First of all, you do not own the car. Under the decriminalised legislation that supports all PCNs, the owner is responsible for PCNs, (appealing or paying), the driver has no role. With paper PCNs served to the car or driver at the roadside, the driver can submit an informal challenge, but if this is declined, the owner is responsible for taking the matter forward to appeal. Of course the driver can pay the PCN if the challenge is declined
Secondly, in your case the owner has paid the PCNs and that is the end of it, no appeals are possible if payment is made. It is possible to appeal if the owner authorises the person to submit representations and also to even take the council to adjudication, but if the case is lost, resonsibility for payment still rests with the owner.