This situation is something that would really annoy me too and I can relate! I hope your gardening efforts have the desired result, with the added benefit of some nice plants. If need be strategically placed sprinklers will also help.
I agree with other comments that as she was immediately defensive, she knew what she was doing was wrong (albeit not in a legal sense). I agree she probably doesn't want to 'mess up' her own grassed area or ruin her underground retic, but she should be parking in her own driveway.
Until recently we had neighbours that used to leave their driveway empty, and park their vehicles on the opposite side of the street, either on the street on the vacant lot, or otherwise park in front of our house. NEVER in front of their own house. They would also hose the dirt off vehicles and trailers in front of our house (after returning from dirt biking). Yet there was no obvious reason why they couldn't park in front of their own house.
They had recently become very garden proud so my take on it was that they didn't want to ruin the view of their lovely yard with untidy looking
vehicles or mess up their 'own' street frontage.
I'm thinking along the same lines about your Mum's neighbour.
Let us know if there are any updates!