If they do send ar tey a claim the POC will probably aver that the keeper is driver, and in any WS trot out Elliott v Loake (criminal so not applicable to civil claims) and CPS v AJH Films, ( Employer employee relationship so again inapplicable) both these have been discredited in court in parking claims, a research on Parking Prankster's blog is very useful, to prepare ammunition if they are silly.
Could point out to Lidl that their parking Cowboy's 90 minute limit could breach the Equalities Act 2010, as someone in your mother's position will need considerably more time than that allowed, also the Planning permission might have stipu;ations about minimum parking time, usually double that allowed say 3 hours. Think ericsbrother will have some ideas here.
Hello and welcome to CAG.
It's very likely that your father won't have to pay these 'fines', but you're going to be doing a crash course with us in how private parking companies work. I hope your father has kept all the paperwork including Lidl receipts? Please ask him not to throw anything away for now.
You did the right thing in contacting Lidl which is what we would have suggested, it's a shame they won't help. I expect someone like ericsbrother will have ideas on how to embarrass them.