Ask an Property Solicitor. Get an Answer ASAP.
The only purpose to a witness is to rebut any suggestion that the document was not signed by the person is purports to bind. Since that does not usually arise as there is no dispute over who was living at the address it matters now.
Can I clarify anything for you?
You don't really need an agreement to be witnessed.
The only point of it is to pre empt an allegation that it was not signed by the person that it binds.
I don't think you need to worry about that. There will be no doubt she was the person who signed.
Why would anybody but her have signed this document.?