Sharon Smith's criticism of Joanne Walder, intended for her 300 Facebook friends, became more public than she intended when her sister reposted her comment on her own public Facebook page, the London Evening Standard reported.
Walder sued, and the High Court said Smith failed to meet the deadline to file a response. The court will decide whether Smith could be ordered to pay damages that could be as high as 20,000 pounds (more than $30,000).
The case is believed to be the first libel action involving private Facebook comments in England.
Smith runs the GI Jane Bootcamp for Women, a fitness center in the Bexleyheath district in southwest London.