My Puppy Has Milk Coming Out Of Its Nose, Why?

I am looking at a 6 week old lab puppy to purchase. The owner said when it drinks milk comes out its nose. She brought it to the vet and they said it does not have a cleft pallet, a heart murmor, or anything. It is a completely healthy puppy except for the fact that it is the runt of the litter. Is this something i should be worried about? What kinds of tests or surguries should be done to figure out what is wrong with this puppy?