I had people asking me if I had FB at work. I told them yes and they asked me to friend request them. I fought it for months, telling them I do not have work friends on it, I am not that a give on it and my life is not that exciting. Finally I gave in because I was told I must have something to hide bla bla bla. I friend requested a few and the original person that said I was being to private decided to no longer be friendly on line and at work. Said that not everyone at work had to be friends and un friended me after 3 months. I guess my life was not as exciting as they thought it would be.
Go with ignoring g the request as long as you can. If this person asks why you did not accept the request say that you have not been active on it and you will try to remember next time you are on.