If her records have simply been misplaced, the original vet should be able to supply you with a copy/duplicate of her records: wellness as well as her shots.
Now, if THAT information was not forthcoming, *then* I'd get all sorts of leery of the transaction. Perhaps it's just as she says: they've moved, there's a sense of unease in the dogs because of it and strange surroundings, and the original dog is trying to exert undue influence over the newer pup. That happens a LOT. Or, you could make the sale pending a clean bill of health from a vet. That, however, could be a bit dicey: I'm one of those who, after holding a pup in my arms, takes immediate possession in my heart
. At that point, I'm lost: my brain (and common sense) have just left the room
Good luck with your decision ...