In my opinion, the sniper on the roof there took the target out as soon as he saw him. I believe he could see the guy interviewed by bbc pointing, but could not see the shooter yet, until the shots were fired. At that point you could see the sniper back off his scope and locate the shooter, reset, and fire.
The real issue here is that that there should of been security on that roof insuring nobody could sneak into a firing position up there.
As soon as you get the alert from the bystander, you call for the USSS detail to get Trump off the stage immediately.