It is important to remember how Framton's opinion of Mrs. Sappleton, a woman he has never met before, has been coloured completely by Vera's account of why the window is open and what supposedly happened to Mrs. Sappleton's husband and brothers one day. Therefore it is understandable that Framton finds meeting Mrs. Sappleton a horrible experience when she talks about her husband and brothers as if they were still alive. Note the following quote:
She rattled on cheerfully about the shooting and the scarcity of birds, and the prospects for duck in winter. To Framton it was all purely horrible.
Framton therefore thinks that Mrs. Sappleton is so deranged by her grief and the tragic accident that happened that she is completely unable to accept the reality of the tragic accident that Vera told him about. Framton's opinion of his host is therefore a direct result of Vera's manipulation, which it is only the reader who comes to realise, as Framton leaves far too rapidly to discover the way he has been tricked.