See a physio, maybe targeted exercise will help. At the very least they will recommend an appropriate support if needed.
ETA like Mijods, another ‘inside out’ suggestion.
Unless there is a contract in place which sets out who is responsible for what, l think the owner would have to accept that it is their responsibility to deal with the consequences.
We have a country park near us that has recently introduced similar charges. So now people park all along the narrow road through the middle. It’s a wildlife trust park, not council, so I do see they have to raise money somehow, but it’s not ideal.
I agree it’s hard for liveries. It’s easy to say ‘chuck him out and let him end his years in the field’ if you own such fields. Otherwise it is usually give up riding altogether as few can afford livery for 2. Some people have the resources to retire, others see their horses as pets, which is...