Thanks for your reply.
As she is a legal owner of the property, she needs to consent to the property being transferred into your partner's sole name, even though she hasn't been paying the mortgage etc.
Likewise, if the property is sold, she would need to sign the Contract, agreeing to any such sale.
You do really therefore need to make her see sense and hope common sense prevails. Although drastic, your partner could go to Court and obtain an Order for Sale, if she was not willing to sell. Perhaps you could suggest to her that this action will be taken if she is not willing to transfer the property to your partner.
Equally drastic would be to threaten to stop paying the mortgage and therefore her credit rating would be adversely affected (but so would your partner's).
There is therefore no magical legal solution to your problem, but I hope I have set out the legal options.
If I have assisted, I would be grateful if you could leave positive feedback.