Unidentified friends of Mills, who received a $35.3 million divorce settlement from McCartney in 2008, said the 40-year-old was outraged when she learned of the suit filed by Sara Trumble, the Daily Mail reported Friday.
Trumble's lawsuit, filed in the British county of Kent, alleges she suffered sexual discrimination, intimidation and constructive dismissal prior to her resignation from Mills's employ three months ago.
Publicist Joe Dolce, who helped repair Mills's public image last year, confirmed the former model felt betrayed by the lawsuit.
"For years she has treated Sara as a member of the family," Dolce told the Mail.
The newspaper said a potential lawsuit payout to Trumble, who served as nanny to Mills and McCartney's daughter Beatrice, would come after Mills has already spent $14.5 million of her divorce settlement.