Free Traffic Legal Advice

Two weeks ago, my mum dropped me off at Manchester T1, went to pay at the barriers on exit, however the barriers were up, didn't notice any signs saying payment must be online, so she carried on. My dad (registered keeper) has now received a PCN from APCOA.

Having read a few of these forums, I believe this £100 fine can be appealed. I'm looking if someone would be able to help me with this please? 

Thanks!!