So, postal PCN received but ignored, then CC arrived adding 50% onto the PCN penalty.
Frankly, there is not a great deal you can do about this. If you wait until the PCN is registered at the Traffic Enforcement Centre, adding another £10 onto the amount, you can submit a Statutory Declaration, but none of the three circumstances you can select from on the form match yours. Your only option is pay-up. Sorry if this isn't what you wanted to hear, but if a recipient of a postal PCN ignores it, then any appeal options are lost.
See here, Form PE3
Form PE3: Challenge an unpaid penalty charge notice.
Edit
I suggest that you contact the council to explain the situation; they may cancel the CC as a courtesy. Your mother can then pay it, or submit representations.
