She was a lost cause. Armine was her opportunity.

Armine had talked to her four days ago of Meyer Isaacson. The Doctor
guessed how, knowing the generous enthusiasm of his friend. And she, a
clever woman, made distrustful by misfortune, had come to Cleveland
Square, led by feminine instinct, to spy out this land of which she had
heard so much. The Doctor's sensation of being examined, while he sat
with Mrs. Chepstow in his consulting-room, had been well-founded. The
patient had been reading the Doctor, swiftly, accurately. And she had
acted promptly upon the knowledge of him so rapidly acquired. She had
"given herself away" to him; she had shown herself to him as she was.
Why? To shut his mouth in the future. The revelation, such as it was,
had been made to him as a physician, under the guise of described
symptoms. She had told him the exact truth of herself in his
consulting-room, in order that he might not tell others--tell Nigel
Armine--what that truth was.

Her complete reliance upon her own capacity for reading character
surprised and almost delighted the Doctor. For there was something
within him which loved strength and audacity, which could appreciate
them artistically at their full value. She had given a further and a
fuller illustration of her audacity that evening in the restaurant.

Now, in the night, he could see her white face, the look in her
brilliant eyes above the painted shadows, as she told to Nigel the
series of lies about the interview in Cleveland Square, putting herself
in the Doctor's place, him in her own. She had enjoyed doing that,
enjoyed it intellectually. And she had forced the Doctor to dance to her
piping. He had been obliged to join her in her deceit--almost to back
her up in it.