Sir Paul McCartney's estranged wife parted company with her lawyers Mishcon de Reya last week, saying she could not afford to retain their services. She was also dropped by spokesman Phil Hall after blasting the media in a series of TV interviews.
Comtesse Michele Elyzabeth, whom McCartney is said to loathe, described herself as one of Mills' closest friends.
She was employed despite telling US TV show Extra that her client had received a court order granting full custody of daughter Beatrice.
The 2006 mistake was later posted on YouTube and mocked on comedy show The Soup.
She told the Mail On Sunday: "I have immense respect for Heather and her work. She is sick, sick, sick of all the negative publicity. I am on my way to dinner now and have to go."