Why is mrsgriffin so sure sue lying about the blond woman in book killing mr griffin?

You're likely referring to the novel "And Then There Were None" by Agatha Christie. In the book, Mrs. Rogers, not Mrs. Griffin, is suspicious of Miss Brent's story about a blonde woman at the inn.

Here's why Mrs. Rogers is skeptical:

* Inconsistent details: Miss Brent claims to have seen the blonde woman leaving the inn at 11:30 PM, which is later than the time she herself claims to have gone to bed.

* Lack of corroboration: No one else at the inn remembers seeing a blonde woman, and the innkeeper specifically states that no woman left the establishment during that time.

* Suspicions about Miss Brent: Mrs. Rogers knows that Miss Brent has a hidden past and is not a particularly trustworthy person.

Mrs. Rogers is not convinced by Miss Brent's story because it doesn't add up and seems deliberately fabricated to deflect suspicion from herself. This suspicion is a key part of the mystery, as it eventually leads to the unraveling of the truth about the killings.