I once had a guy that would do the same to girls when we were walking in town or anywhere really and it bugged me but one day i caught him staring after this girl with a really nice back side and legs and just said "wow! she's fit! Good choice spotting her!" and then after that he didn't do it so much! Guess he had never really noticed that i knew he did it or something so try that with her i know you won't want to sound 'gay' or something but just comment on his really nice jumper or something or even about how his jeans or something are similar to ones you own or something, maybe it'll work maybe it won't but its worth a try!