What factors show that Framton's visit to Mrs. Sappleton might have worsened his nerve problem?

Framton Nuttel is an example of a character who is created to suit the purposes of the author's plot. Young Vera's practical joke would not be as effective on a man in normal health. Saki takes pains to emphasize that Framton is only here at the Sappleton's because of his bad nerves, or neurosis, as we would say in modern times. He was told by his doctors, who were in complete agreement, that he needed a "rest cure" in the country, and then his sister gave him some letters of introduction which brought him to this zany country establishment. The whole point of Saki's story is that Framton's nerve cure only made his nerves worse. 

"The doctors agree in ordering me complete rest, an absence of mental excitement, and avoidance of anything in the nature of violent physical exercise," announced Framton, who laboured under the tolerably widespread delusion that total strangers and chance acquaintances are hungry for the least detail of one's ailments and infirmities, their cause and cure. 

He is supposed to get complete rest, yet he ends up running for dear life down the country road. He is supposed to have an absence of mental excitement, yet Vera provides more mental excitement than he has ever had before when she makes him believe he may have three ghosts with shotguns chasing him. He is supposed to avoid anything in the nature of violent physical exercise, yet he disappears from the story like this:

Framton grabbed wildly at his stick and hat; the hall door, the gravel drive, and the front gate were dimly noted stages in his headlong retreat. A cyclist coming along the road had to run into the hedge to avoid imminent collision.

So if Framton's doctors are correct in their diagnoses and prescriptions, his nerve problem must indeed have been worsened. 

It is probably significant that he didn't say anything about his nerve problem to Vera. If she had known about it, she might not have made him the victim of her ghost story. Not that she would have been too much concerned about Framton as that she would have been concerned about herself if their guest died of fright.

Vera is another example of a character who is created to suit the plot, as is Mrs. Sappleton. Vera is a very "self-possessed" young lady. She is just old enough to be believed and just young enough to think of pulling such a trick. When she deliberately loses her self-possession, the contrast between her tranquil demeanor and her look of "dazed horror" is so effective that it is responsible for making Framton panic and flee into the night. Mrs. Sappleton has become almost as nutty as Nuttel. She hears nothing but male talk about shooting birds, and so Vera knows she will talk about that subject when she meets her guest.

She rattled on cheerfully about the shooting and the scarcity of birds, and the prospects for duck in the winter. To Framton it was all purely horrible.

Vera is not only a creative story-teller but an actress and stage director. She even knows that Ronnie is going to burst out singing, "I said, Bertie, why do you bound?" How could Framton suspect he was being set up? Even if he should find out that Vera's story was a complete hoax, he may never be the same again.