If the jaw is in fact undershot (bottom teeth coming up over the top teeth) then this can only worsen as the bottom jaw grow. It's the reason that puppy bites that are only just scissor will go undershot, and with a puppy you want a really good deep scissor bite, in hopes it stays scissor.
Totally correct - this is a big no-no in my breed (nothing but a scissor bite is acceptable). In the worst scenario, a bad bite, with a big gap, either way, will affect the ability to eat, and may well lead to dental problems later on because the bite isn't meeting correctly. And of course, a significant overbite may well hinder a bitch from dealing with her puppies, on whelping (nipping the umbilical). Obviously dogs with incorrect bites should be excluded from any breeding programme because it is a significant fault.
If you are intending to show, don't buy this puppy - you won't be placed. I had one who went into quarantine with a pretty 'tight' bite. Edge to edge is accepted in N.America in my breed, but NOT here. By the time they came out of quaranting, he had definitely gone undershot. Tragic because he was otherwise a lovely dog - which is why we ran him on with two of his sisters. I didn't like the look of his bite early days (his dad was edge to edge but I risked using him for his other qualities I needed, and I'd not had this fault in my line up to them).
As a pet only, this puppy may be absolutely fine, depending on how bad the fault becomes later on (feeding). His breeder should have been aware without having the vet tell her btw.