When does Miss Strangeworth stop calling Mr. Lewis "Tommy?"
In "The Possibility of Evil," Miss Strangeworth stopped using Mr Lewis's name, "Tommy," on the day that he left high school and started working in the grocery. Likewise, it was also at this time that Mr Lewis stopped calling Miss Strangeworth by her first name, "Addie."
Evidently, for Miss Strangeworth, Tommy's move from high school to the workplace symbolised the end of their childhood friendship. In her mind, Mr Lewis was now a respectable member of her town and should be addressed accordingly.
Moreover, in referring to the fact that Mr Lewis is in the grocery while Miss Strangeworth is "living alone" suggests that a romantic connection had once occurred and that Mr Lewis's move to the grocery brought it to an end. By calling him "Mr Lewis," Miss Strangeworth, therefore, reinforces the end of his connection and the beginning of a more formal and business-like relationship.