Op,
in general the law splits people into two groups when it comes to financial things.
first group, people who are not experts. A lot of consumer law is about protecting people and making sure that financial products and legal things are explained properly to people who are not experts and that these people are advised correctly by lawyers and financial experts.
second group, experts. These are people who know what they are doing and are able to buy more complex financial services and get involved in complicated legal things because they are experts.
it sounds like at some point someone who owned your mum’s home (maybe your dad, maybe a previous owner) sorted out the freehold/leasehold situation with next door. This person would have been an expert - they would have understood what they were doing.
you are now in a situation where because your mum holds the freehold (which is a complicated legal situation) she is assumed to know what she is doing and the solicitors for the neighbours are allowed to send her these forms and expect them to be filled in.
the problem from your point of view is that she isn’t an expert and neither are you.
you do unfortunately need actual legal advice as simply by owning this complicated legal right you are presumed to know what you are doing.
it’s possible citizens advice may be able to help (as clearly you are not experts) but it’s also possible that as this is legal stuff that is not in group 1 (ie you are a not expert who should have been advised correctly by experts) they won’t help.