Any official Lab rescue would have insisted they sign the dog over, and in that case there is no comeback. Without knowing the people or the details it is difficult to know but surely break ups don't happen overnight, they must have been considering it and who knows how long they will stay together. It is a drastic step to rehome a dog, might have been better to board it till they knew where they were. The rescue people will want what is best for the dog, and they need to consider how permanent the reconciliation is likely to be before unsettling the dog again.