Mortgage rate In practicality jumbo is like 7.0% 4 days ago...
You haven't given enough info. in reference to everyone who is involved in. Do your parents have a living trust set up? How long has he lived there? Info. to determin net proceedings, etc. Assuming GF stands for Girl Friend not Grand Father, you should also consider proper vesting in case things get sour.
It sounds like the first step would be get your parents permissioin. Secondly,
consult professionals who are familiar with local laws such as a good estate lawyer, an accountant, etc.. You probably wouldn't want to involve realtors but you should still use an escrow company.
Good luck. Most cost effective thing will be living with your parents.