While it is possible that a single 'grandmother neuron' exists in the brain, it is more reasonable to expect that many neurons are responsible for identifying whether the brain is experiencing 'grandmotherness'. Just because the concept of 'grandmother' is a single well-defined label doesn't mean that that is how it is represented at the neural level. The brain's mind's eye is not a disconnected observer of the activity of neurons, it is embedded within the observation itself.
Claims that activating a single neuron can trigger specific 'grandmother memories' may be valid from the point of view that 'stimulating this neuron causes this psychological reaction', but this scientific observation doesn't prove that the single neuron represents anything in particular - merely that it is part of an assembly that in some way leads to an overall impression. More intuitively, a specific impulse to a neuron as akin to dropping a marble at a specific place on hilly terrain. The dropping position in crucial, but so is the interaction of all the other neurons.
Ideas as Music
Another way of picturing how the brain's parts may not map 1-to-1 with concepts on a neural level is to think of an orchestra playing a specific chord. A specific note from a specific instrument may rule out a great many different potential chords, however that one note doesn't predetermine the overall effect, nor is it the only way to acheive a given outcome. The chord produced emerges from the orchestra, the product of all the performers playing their part.
As far as the operations of the brain are concerned, embodying 'grandmother' as an ensemble effort has the advantages of being robust, maleable and naunced.