One single grape is unlikely to kill a dog as it usually takes more than one grape for the dogs kidneys to be affected by the grape which is what causes the death.
My dog accidentally ate a few grapes before and I took him to the vet and the vet said that he would be just fine but that if he ate more than a few grapes then it could be a problem.
But the vet said that just one or two grapes is unlikely to cause any problems with the dogs kidneys or health so I wouldn't worry if your dog just ate one grape.
Just keep the dog away from any future grapes but accidents do happen.