He is typically circling the perimeter of her life and does things like pay for her dinner, drive her kids to shcool, or flirt on her Facebook page in order to "keep out" the attentions of men who would actually like to date or touch her. Sometimes he even writes songs about her. He is beyond protective, discreetly jealous, and has the ability to swoop in and destroy the possibility that his single woman friend will become involved with another man from any distance. He is the ultimate cock blocker.
See also: cock block, cockblock, cock blocking, cock blocker, cocknlocker