For a friend- is this considered to be a serious issue in a pet dog- it is a KC red'g dog and pup is healthy but the top jaw overshoots the bottom when you look inside the mouth.
As long as it isn't so overshot as to cause problems eating then I wouldn't bother too much in a pet dog, depending on the age of the pup it could improve anyway as the jaw grows and strengthens. I can't say for all breeds but an overshot mouth is considered quite a serious fault in the show ring for GSDs.
In some breeds it is normal for a puppy to be overshot..bull terriers for instance..and this will correct itself as the head and jaws grow to adulthood.Only if the jaw is overshot by a large amount in an adult (by an inch or more) might there be any problem. Referal vets would of course say the opposite ,frighten puppy owners half to death ..and make lots of filthy lucre .