If you follow the links here it will tell you the type of evidence that is needed for her to be able to get legal aid.
https://www.justice.gov.uk/private-family-matters-legal-aid
Yes, I agree Quoi, it is wrong, very very wrong. I have been on the receiving end of these type of allegations myself so I really do understand your frustration. I just hope that public funding is refused and that your son can use some of the saved CSA on enjoying some time with his child.
As Eddie says, the family courts have seen all this before - the term parental alienation is not liked by the Courts but instead prefer "implacable hostility" - might be worth looking some of that up.