Unless he's putting his bitches up in above-ground pens while they're in heat, he is basically leaving them as "open bait" for the stray males in his area and inviting this to happen. So I would personally term this negligence rather than a "mistake."

It is my understanding that abortive techniques on dogs are somewhat dangerous to the bitch, but I have not really studied this aspect of reproduction since (through preventative measures) I have never had this happen to one of my bitches.

You might want to consult your local vet for more specifics.