'This Is Not Good': Your Best Friend's Boyfriend With Another Woman

Feb. 22, 2008— -- Imagine that you're dining at a fancy restaurant, when suddenly, across the room, you see something that makes you do a double take. You see your good friend's significant other with a complete stranger ... and it's clear dessert isn't the only thing on their minds.

Would you tell your wronged friend?

ABC News recruited six couples and asked them to pretend to cheat on their mate in front of an unsuspecting best friend.

We set up our experiment at Ennio & Michael's, an Italian restaurant in New York's Greenwich Village. The restaurant was rigged with hidden cameras. Outside in a nearby control van, we watched with Donna Barnes, a life and relationship coach, as the ethical dilemma unfolded.

Watch the premiere of the series "Primetime: What Would You Do?" Tuesday, Feb. 26 at 10 p.m. ET

Our first participant was David Woody. He and his girlfriend, Mary Ellen Jarrell, are a devoted couple who had just moved in together after four years of dating.

At the beginning of the experiment, we sent David into the restaurant with another woman on his arm -- an actor hired by us. The two canoodled affectionately at a cozy corner table.

Minutes later, Kiley, Mary Ellen's best friend, walked into the restaurant. She settled at a table across the room with our accomplice, Adrienne. Unlike everyone else at the restaurant, she had no idea that she was about to see her best friend betrayed.

Just before our experiment, we interviewed David and Mary Ellen to see how they thought Kiley would react.

"I don't think she'll say anything at all," David said, and Mary Ellen agreed. "She knows how happy I am with him," Mary Ellen said. "So she will think if she tells me she will devastate me and ruin my life."

Were they right? We were about to find out.

Adrienne prompted Kiley by asking her if the man in the corner with the beautiful blonde looked like David. Kiley said she thought his hair was lighter.

But when her friend left for the ladies' room, Kiley looked again. And again. She strained her neck to see if it was really David with another woman. Her eyes widened and she began to shake. She sunk down into her chair.