Susan Braithwaite of Selkirk had been in an open relationship with the man in question after the end of her 36-year marriage.
He started a new relationship with another woman in September 2019, but the court heard that he had continued to be "intimate" with Braithwaite.
Selkirk Sheriff Court was told the new woman was unaware of Braithwaite until she received a telephone call from her on June 2, 2020.
Braithwaite told the woman that the man had "been intimate with both of them during the same period" and said that she could provide proof by viewing a video she had.
However the woman declined to see it.
The woman then confronted the man who denied it.
She then contacted Braithwaite and asked her for a screenshot to prove what she was saying.
Depute fiscal Joanna Waller said: "She sent a screenshot showing the accused and the man having sex."
Braithwaite pleaded guilty to a charge of disclosing a photo showing a man in an intimate situation without his cosent and having not been previously disclosed.
But she denied a charge of voyeurism by filming an act of a person in an intimate situation with the intention of showing another person.
Defending Braithwaite, her lawyer Liam Alexander said that when Braithwaite goes on holiday she switches on security cameras in her bedroom. He said that she had forgotten to switch them off on her return.
The court was told that when she was asked to provide proof of her relationship with the man she looked through previous footage and found the sex scene.
Mr Alexander added that his client had only been trying to point out to the woman what kind of character her new partner was in that he was having sex with both of them at the same time.
Sheriff Peter Paterson said that in the "exceptional circumstances" he would grant first time offender Braithwaite, of Chapel Street, Selkirk, an absolute discharge meaning it would not count as a criminal conviction.