There are a few possibilities that come to mind-

1) The vet mistook some other scar for a spaying scar. Is it possible your friends might not have recognized the signs of a dog in heat?

2) Sometimes other things, like some kinds of bladder infections, can have 'symptoms' similar to a heat cycle.

3) Sometimes they don't remove all ovarian tissue when spaying, either because the dog's ovaries were abnormal, or the spayer was sloppy, or whatever. So she could still have a heat cycle, though she wouldn't be able to breed succesfully.

I have heard of these happening through working with shelters/vets, but have never experienced any. Sorry I can't help more.