It's sort of assumed that the one controlling the sentry is dead at that point (because the spy sneaked behind him and killed him, as that's what spies do. )
It depends on the location. I've had times when a disguised spy has run right up to me in plain sight, sapped the sentry, and whipped around and backstabbed me before I could fire enough shots to kill him even though I started shooting him the moment I saw him approaching and suspected he was a spy. So basically this gives sentry campers a sappable submachine gun to replace their pistol.