
I would suggest that if the people are insistent that she calculates 4 years worth of boarding charges!! At even £5 a day that would come to over £7000, less what they paid for the dog of course. I am sure if that was pointed out, they would soon decide it was better to get a new pet now that circumstances have altered!!
A friend in my breed had a similar conundrum, but in this case the people wanted the dog back from the begginning once their problems were sorted. He had been shown as a puppy and won a CC, so the breeder agreed to keep him for a while until they could have him back. He gained another CC, but the time stretched on with just visits bearing gifts (the breeder kept the dog out of her own pocket). This went on for two years, by which time he was a veteran. The breeder finally gave the people an ultimatum, as she didn't want him to spend another winter as a kennel dog (which he had to be due to her owning other males). He has now been happily homed as a spoilt only dog with experienced breed folk who wanted an older dog.
Sometimes people only think of how they feel, and not what is best for the dog! this one had always been a pet, and had to be a kennel dog for two years because of selfish owners, whereas the breeder could have rehomed him almost straight away.