Ms Francis came up with a code word, 'miley', which she used to remind Ms Highley to smile.
The authority decision says this made Ms Highley "very upset and nervous as staff would say this to her even when she felt she was smiling".
Ms Highley was said to have cried for a long time because she was disappointed with herself and she was not improving as much as she thought.
In January last year, Ms Francis met Ms Highley again and reiterated she was "not smiling enough" and "her job could be in jeopardy".
Ms Francis also claimed customers complained about Ms Highley, describing her as "unhappy, bland, lacking personality, looking miserable and seeming to hate her job".
After all this, Ms Highley was relieved when she finished her 90-day trial period -- however, she was dismissed a day later.
According to the authority, the dismissal was because Ms Highley did not "smile enough" and Ms Francis did not see any improvements throughout the trial period.
Authority member David Appleton agreed it was likely Ms Highley's customer service skills were not meeting the requirements of the employer.
However, he said Ms Highley was "ambushed" because she was only given four days to improve before her trial period ended.
Ms Francis also claimed Ms Highley lied about having a full duty manager's certificate, restricting her ability to do her job.
Mr Appleton said this was not why she was fired.