Is there any way I can help my girlfriend buy her first house?
Unless you are planning to marry this girl, I would say try to protect yourself legally as much as you can. I am a firm believe in not mixing business and family members or your significant other unless you are legally married.To be legally linked to the property you would have to be a co-borrower...

Should I buy an investment property?
^ I agree with Tim. It seems like the best way to make a good ROI.