What is the best way to not be seen on Facebook? (please see details)

I am facebook friends with a former roommate, who used to date my friend. He got kind of stalker-y with her. I want to be facebook friends with her, but I don’t want the old roommate to be able to see my friend. How can I be friends with both of them but protect her from him? Should she block him? Would he know she’s blocked him? I’m thinking this would be easiest, but is there a way she could set her privacy to achieve the same results?

As an aside, I’m in my 40s and can’t believe I’m seriously asking this question. Crazy.