How do Mr.Frank and Taplow react to the sudden arrival of Mille Crocker?

Frank was goading Taplow into sharing the Latin joke that Crocker-Harris had cracked in class and Taplow seemed hesitant to do that. But Millie's arrival put that in check and Frank was concerned that Millie might have heard them talking about her husband. They could not be sure if Millie had heard the conversation that had passed before she came in and Taplow was worried that his remove would be gone if Millie reported him to her husband.