Hello, welcome to the website. My name is Tony, I can assist you with this.
The only way of doing it obviously is with the lender's consent. If your girlfriend has a decent credit position, then I would have thought the lender would allow her on to the mortgage. What they will not wan to do, is release somebody who has good assets etc., if they are concerned that there might not be enough equity in the property should they need to sell it in the event of default.