Tenant who signed agreement and never moved in

Other

WE have a house where the existing tenants found a person to take a room and they arranged the viewing and they witnessed her sign the agreement. She was added onto their Joint tenancy agreeement.
I had problems conmtacting her and she never paid any rent. When I finally tracked her down she said she never moved in and couldn't remember signing the agreement. I have the agreement with her signature on but she says the tenants in the house did not explain to her what she was signing.
She is in the process of taking lrgal advice as though she know agrees she signed it, she says she doesn't know what she was signing.
Where do I stand with this?
Thanks