I've read that a simple Gunn diode will do this if the proper resonantor is applied. Appearantly those are used in a lot of motion detection systems like utomatic doors and high-end security systems. Note however, that --and here it comes, the obligatory legal warning; all together now...
"radar activators" are illegal in certain states. Check your state penal code.
...Now with that out of the way, I should add that I heard this many years ago from an unprofessional source. Also, the source mentions this only affects the X band equipment which would leave you out in the cold if your friend's detector is designed for the most popular K or Ka bands. Additionally, they cliam you have to be moving toward the radar detector because of a small bandwidth difference.
In a nutshell: the source is seedy, the technique is creud, but it's just about crazy enough to work under a blue moon.