I have long haired and short haired cats and each year in springtime I take the longhairs to the vet for sedation and a close clip. If I don't do this, their hair becomes horribly matted and tangled, and pulls on their skin, causing redness and soreness. No amount of brushing prevents this. Indeed, the cats hate getting brushed and a once a year clip is much less stressful for them.
I am not sure why the short haired cat would be clipped. However, some short haired cats also get matted fur, if their fur is particularly dense. But that is much more easily dealt with by brushing if their hair is short.
Why not bring it up in conversation with your neighbour, and mention that you wondered what the advantages are of clipping the cats' fur.