OMG!!! What a serious temperament flaw. Are you refunding the full purchase price, and paying for a private behaviorist, and providing them with twenty weeks of dog classes, and suggesting an immediate spay, and providing three free sessions to the dog psychic for the poor lab???
Have you told them to schedule an appointment to have a complete blood panel done with a thyroid and vitamin B check, and have you suggested they get her off the sugary kibble they must be feeding???
Or are they feeding her RAW food? That is probably it, you know how that makes them aggressive.
It just goes to show what happens when people buy dogs from breeders instead of getting dogs from rescues and shelters. A shelter dog would NEVER behave this way as it would know it was on its second chance and would be perrrrfect.