
It's not uncommon in my breed. Nothing to worry about. Can look a bit unattractive, thus not desirable in a show dog. Nor in a dog you want to breed. But the dog will have no ill effects, and it's definitely not anything which needs treatment.
Since the dog in question is bought as a companion dog, no harm done. Maybe the breeder didn't notice? The ribcage is very soft in young pups, it's quite possible she didn't realize this was something out of the ordinary.
Enjoy the pup, and don't worry. No need to do anything about it at all.
HTH
Karen