This returning a call episode reminds me of what happened in P&P when Caroline Bingley formally returned Jane Bennet's call in London. Caroline was not happy when Jane called on her, but she was perfectly civil and meant to keep up appearances. But Darcy decided that Bingley was not sufficiently recovered from his love for Jane to be able to see her without danger, and to conceal Jane's presence in London from Bingley, it was necessary to drop the acquaintacne, or at least make it clear that it was only a formal acquaintance, otherwise Jane would call again, and sooner or later would run into Bingley. So Caroline only stayed for 15 minutes, and was cold and formal with Jane, so that Jane felt slighted and decided to drop the acquaintance.
Here I see Emma in the role of Darcy, and Harriet strangely combining the roles of both Caroline and Bingley. Poor Harriet suffered more though -- she was so torn between what she felt was due to Emma and what she felt for the Martins.