A person's parenting ability is not necessarily reflected in their children; great parents can have problem children, and high achieving children can come from broken homes. In light of this, any parent's advice on how to deal with anybody else's kids has little meaning solely because they have kids - their own experiences might be irrelevant in a given situation, while a childless person might have second hand knowledge pertinent to them.
Good advice can come from anywhere, even a childless adult, assuming the childless person didn't grow up to adulthood 100% alone. Everyone tends to parent like they were parented. Also, assuming that the childless adult wasn't mentally and physically abused to the point that they have unhealthy cognitive abilities and sociopathic behaviors. Basically, anyone that was raised in a healthy social environment has the potential to give solid parenting advice.