When does Bob realize that the man to whom he is talking is not Jimmy Wells, and how does he react in "After Twenty Years"?

Bob realizes that the man with whom he talks is not Jimmy Wells after they walk under the brilliant electric lights of a drug store. He is startled that this man is a stranger, and he exclaims, "You're not Jimmy Wells!" Bob then points to the man's differences from his old friend.

In what once was the doorway of his and Jimmy's favorite restaurant, "Silky Bob" has waited twenty minutes for his old friend after having spoken to the beat patrolman. Then, he sees a man with his coat collar turned up approaching him. "Is that you, Bob?" the man asks. "Is that you, Jimmy Wells?" calls out Bob. After asking how the other is, Bob accompanies the taller man toward a place that is known to Jimmy; as they walk along arm-in-arm, Bob relates "the history of his success." But, when the bright lights of a drug store strike Jimmy's face, Bob suddenly stops, realizing he has been tricked.

"Twenty years is a long time, but not long enough to change a man's nose from a Roman to a pug."
"It sometimes changes a good man into a bad one," said the tall man. "You've been under arrest for ten minutes, 'Silky Bob.' Chicago thinks you may have dropped over our way and wires us she wants to have a chat with you."

The plainclothes policeman informs Bob that Patrolman Wells has asked him to give Bob a note; also, he tells Bob that before they go to the police station, he may read the note under the bright lights. Bob's hands shake as he reads this message from his old friend, who could not make the arrest of "Silky Bob" at the time that he talked with him in the doorway.