Getting the girlfriend off the deed is the big issue for your son. A deed will require her signature, so as long as you can get her signature, then that should be the main goal. The mortgage can be paid off at the time of the transfer of her interest to him. Unfortunately, since they are not married or formerly married, the deed transfer in PA will involve some transfer tax. In most areas the tax is 2% but in certain cities it is as high as 4% of the property value. It would only be 50% in this case of the value of the property since your son holds 50% of the title to the property. Don't pay off the mortgage until you have a signed deed from her. The deed can be for a dollar. You should get a real estate attorney to help you through this.