Answer:
When Mrs. Sappleton told him that she was waiting for her husband and her two brothers who had gone hunting.
Vera stared out through the open window and pretended to see something horrifying and unbelievable.
Step-by-step explanation:
Firstly, Vera convinced Frampton that her story about Mrs. Sappleton's husband and her brothers and why the window was always kept open was all true when Mrs. Sappleton told him that she was waiting for her husband and her two brothers who had gone hunting.
Secondly, when Mrs. Sappleton's husband and her brothers returned home for real, Vera stared out through the open window and pretended to see something horrifying and unbelievable.
These two instances completely convinced Frampton that Vera's story was true and he felt the need to escape from there as soon as possible.