She claims Ms. Mills treated her in a "humiliating and demeaning" way by replacing her as nanny and asking her to perform cleaning duties, and that her ex-boss has a "distrustful and unpleasant" side.
Ms. Mills denies this. She says her relationship with Sara Trumble turned sour after she refused to pay £4,000 for her to have a breast enlargement operation.
On cross-examination, Ms. Mills, told the tribunal: "I didn't criticise her wanting to have a breast enlargement, I just felt it would have been wrong of me to help with something that could have been a reaction to her boyfriend or her hormones."
Nick Fairweather, counsel for Ms. Trumble, accused Ms. Mills of fabrication, saying: "I suggest that your previous comments were to intimidate and bully and belittle her to stop her from going on with this claim. I suggest that you are telling a very, very unpleasant lie."
Ms. Mills responded: "I'm not telling a lie. She asked me for £4,000. How on earth would I have known that amount when I put my statement together?"
To which Mr. Fairweather said: "If this tribunal finds that you lied about that, would you agree that this is a despicable thing to do?"
Ms. Mills replied: "I never did it."
Following the hearing, Ms. Mills said: "It saddens me greatly that this case had to come to court. I treated Sara like my own daughter. I felt I did everything I could to help and support her, both financially and emotionally.
"And when she became pregnant, I was really excited and pleased for her. I spoilt her like my own daughter, lavishing gifts on her. So to hear the lies that have come out in court has hurt me beyond belief. I am absolutely devastated. I feel as though I have been betrayed by someone who I felt was part of the family."
The tribunal is expected to deliver its verdict by the end of May.
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