I've seen mirrors used to give the spanker or spankee views of the proceedings in ways they might not otherwise get to . . .enjoy. However, I haven't seen much that would compare to the sinking feeling of catching a glance of an unexpected observer.
Knowing her friend would witness the spanking was bad enough, but knowing she was watching this step- even though there was no indecent exposure, no kicking or wailing- was almost worse.