There is a difference between a charity creating a valuable asset with the specific intention of gifting it to a public body (i.e. a new skatepark) and a charity making a financial contribution towards what is, in effect, a fundraising campaign to improve access.
I've assumed that the charity in this instance is either a grant-making charity or a charity that exists (or simply has the power) to provide facilities for people with disabilities. It is unusual, although not unknown, for a grant-making charity to be so proactive. In either case, the benefit is to the users of the path, not to the Parish Council and, from the description provided, it appears that this is not a project which the council would have any legal obligation to undertake.