The R does not blow the whistle under most circumstances.
As a back judge, I will generally blow as the kick is approaching the uprights, so that I'm not drowning out my line judge potentially saying "Yes" or "No".
If the kick does not clear the line and approach the uprights, then it's not my whistle. In that case, the R can kill the play, but it must be obvious the kick has failed. You don't want to be an eager tweeter -- holder fumbles the snap, kicker swings his leg, but misses the ball... the play's not over, don't be overeager to kill it.