Good luck op.
We have a similar problem with a nightmare elderly neighbour next door. I think she too has mental health issues. I did feel very sorry for her (and still do really, altho there’s a lot of frustration there too).
Her fence came down this winter. Her garden is a JUNGLE of brambles and nettles (to the point the council came and gutted it a few years ago due to a colony of rats!). They have put a charge on her house for the costs. She is a horder. The house is falling into a state of disrepair. The garden once again looks as it did before. She owns the house outright after inheriting it.
She has a huge tree which has grown into the fence line pushing it over and into our garden. Two panels have also come down in the wind and are completely rotted and need replacing. So even if we wanted to replace the fence at our own expense we can’t without removing the tree (quoted £1.5-2k due to its size and how close it is to both houses). There’s no way we can afford that.
The council just aren’t interested, despite saying in 2020 that they were concerned- the fire service visited and said due to the hoarding it was a health and safety issue. The lady from the council who came to check on the garden work said they were going to get involved with partially clearing the house because of h and s (boxes stacked floor to ceiling so if there was a fire she’d be stuck and it would go up like kindling).
But nothing has happened since. I’ve tried chasing it up multiple times, also explaining that the garden situation is as it was when they got involved previously. I was passed pillar to post, nothing happened. Last person I spoke to said that ultimately it’s up to the individual how they live their life. I think they’ve just run out of money.
We’re completely stuck. The council have basically shrugged their shoulders and said there’s nothing anyone can do.
As a temporary measure could you block it off with some pallets? That is what we have had to do. It looks awful but stops our dog wandering into her jungle.