Olivia Kopple was the mistress of Gary Freleng.
Background
Olivia and Gary's wife, Deb Freleng, were friends. Gary and Deb were in an open relationship, allowing Olivia to be with Gary.
One day, Olivia goes to meet Gary in the park, despite a previous incident. The two hold hands and smile at one another. Suddenly, Olivia hears a heart beating loudly, and realizes there's something wrong with Gary. Gary tears open his shirt to reveal his heart literally beating out of his chest. Olivia screams as Gary's heart beats harder and harder, until it explodes out of his chest, covering her with blood.
The incident draws in the attention of Sam and Dean Winchester. They initially believe that Deb killed her husband because he was dating Olivia, but the two men find Olivia and Deb comforting each other, revealing the nature of Gary and Deb's relationship.