On the third day of an employment tribunal brought by Sara Trumble, 26, Ms Mills said they were "very close", but their relationship turned sour when she refused to give Ms Trumble 4,000 to undergo breast enlargement surgery in the spring of 2008.
Ms Mills said: "I had a breast reduction when I was 21, and I couldn't pick anything up for weeks because it was so painful. 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."
Ms Trumble's solicitor, Nick Fairweather, accused Ms Mills of making up the issue as a way of explaining why Ms Trumble turned against her.
He said: "I suggest that you're telling a very, very unpleasant lie."
Ms Mills denied his suggestion.
A reserved judgment will now be made, with the verdict expected to be given by the end of May.