Stuart Billingham, 52, a Prison Officer at Shrewsbury jail, sent letters to the airline calling Nathalie Ehlinger, 27, "a liar and a cheat". He distributed copies of a similar letter to ground staff and the public at Stansted airport, and to shoppers in Bishop's Stortford, Hertfordshire.
Mr Billingham, of Severn Meadows, Shrewsbury, who represented himself, denies two charges under Section 2 of the Prevention of Harassment Act relating to the letters.
Ms Ehlinger, who now works for Air UK at Stansted, was close to tears as she testified. She told the court that he had put her through "four years of hell".
"I feel really awful. I have never cheated and I have never lied," she told the court.
David Rothera, for the prosecution, said Ms Ehlinger began a relationship with Mr Billingham, who has two children, after beginning work as a live- in au pair in 1992.
After a year she returned to France to take a course with the intention of becoming an airline stewardess, starting work for Air UK in 1995.
The letters that Mr Billinghamsent her employers last year claimed she had lied about her health and other matters to get her job. He accused her of being willing to "tell as many lies as it took to get the job".
Mr Rothera said the accusations - which included that Ms Ehlinger suffered serious problems with a knee - were untrue. Ms Ehlinger said she had once had her knee checked by doctors after suffering a fall, but that it was found to be perfectly healthy.
Ms Ehlinger told the court that Mr Billingham obtained a court order against her for pounds 46 for returning her belongings to France. She paid the money because she was "frightened."
But it was the letters to her employer and those distributed at the airport which had upset her the most. "I felt very humiliated. I've been going through hell for the past four years. I was scared of losing my job," she told the court.
During Mr Billingham's questioning, she said: "I wanted you to stop writing the letters and to stop harassing me." The trial continues.