It might have been an idea to point this out to her when she was doing the work, because at that stage, you could have got an emergency injunction to stop her.
Assuming that the ground, which he removed and your ground, which is now exposed has been there for more than 20 years, then you have the right support from her land and he/she wishes to remove something which removes that support, she is faced with making arrangements to retain your land. After 20 years, you were probably acquired a prescriptive easement/the right to have your land supported. To exaggerate, she cannot excavator quarry right next to your land and allow your land to fall into it with impunity. The situation with the bank, which she removed is no different.
The situation with the fence is completely the opposite. She's not allowed to build up her land and expect you to support it. If the fence is yours or joint and it rots.she is liable repair it . If you can improve that it is rotted through her negligence, piling dirt against it. Using a similar analogy to the quarry, she cannot put a slag heap on her land which spills over onto yours (as would happen here) and expect you to retain it.
There is a further issue here, and that is if she builds a retaining wall, she can only build it on her land if you do not give consent for it to straddle the boundary.
So, there you have the legal situation. Check your house insurance deceive you have legal expenses cover which would pay for any legal costs. Get a solicitor to write telling her to make arrangements to support your land and to make arrangements to support her own land, failing which you reserve the right to make an application to court for an injunction to compel her to do it, for damages for the trespass of any soil, which goes over onto your land, for damages in respect of the rotten fence and court costs.
On these facts, she simply cannot do what she has done.
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She should not have attached the feather boards in the first place, any nails into your posts are trespass and it is potentially criminal damage if she knew that she should not have done it. However, that does not give you the right to remove them, but it does give you the right to make her remove them, with a court order, if necessary.
With regard to your returning the garden to its current state after maintenance, that would be the case had she not done what she has done.
If he is entitled to remove the support from part of your garden, there should be no reason why you cannot remove it from hers. In law, that does not work, of course, because just because she's done something does not mean that you can.
I would try one more letter and if that fails, your solicitor needs to write the snottiest letter that his computer will generate.
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