'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.

Soon she discovered a mirror we arranged on the wall to give her a perfect view of the cheating couple. A column blocked her from being seen by the cheaters.

When Adrienne returned, Kiley told her she thought the man was David, because he had the same mannerisms.

"She's in a painful situation," Barnes said. "She knows how happy her friend is."

Barnes emphasized that Mary Ellen had just moved in with David and rearranged her whole life around him.

"Now he's with some other girl, [and] that's horrible information to be the keeper of," she said.

To add more pressure to the dilemma, we included a twist. Kiley's best friend, Mary Ellen, ,who was being cheated on, made plans to join her for dinner at the same restaurant. Kiley anxiously anticipated her arrival and shared a secret with Adrienne.

"OK, I'm going to tell you something about Mary that she doesn't tell anyone," Kiley said. "But she was married before and he left her for another woman, so this is not good."

Barnes warned not to assume that your friend would want to know.

"I mean a lot of couples ... do have open relationships," she said. "Maybe that's their thing. Maybe they're swingers. Maybe, who knows, they just might not feel comfortable to fill you in on all their little personal details."

Kiley felt conflicted about telling Mary Ellen. Adrienne suggested that David and the woman were merely friends. Kiley started drumming her fingers on her chin, threw nervous looks and sipped her wine.

At last, she decided to make sure it was really David. She fired a warning shot by text message. Her text read, "If it is you, I'm here and Mary's on the way."

As soon as she sent the message David reached for his cell phone. Through an earpiece, we told our cheaters to look embarrassed, get the check and head for the door -- right by Kiley's table. But Kiley did not even look up as he walked by and left the restaurant.

We asked our expert if texting was a good idea.

"I think it's an excellent idea to text him because it's going to confirm yes it is him, or no it's not," Barnes said. "It's not causing a scene. It's just the safe way to go about it."

Kiley's dilemma compounded as she remembered Mary Ellen was about to arrive. Adrienne asked Kiley what they should do. Kiley said she did not know, and decided she needed a lifeline and a cigarette.

She walked outside with Adrienne and called her mother. She explained the situation and asked for advice. Her mom told her to tell Mary Ellen, but on a full stomach.

They returned to the table and ordered more wine. They expected Mary Ellen any minute. Adrienne asked what Kiley was going to say when Mary Ellen arrived. Before she could answer, Mary Ellen walked through the door with a big cheerful greeting and kissed Kiley on the top of the head.

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The pain in Kiley's eyes spoke volumes. Clearly nervous, she took a deep breath, exhaled and looked straight into Mary Ellen's eyes.

"Mary, we have to talk to you — it's serious," she said. "When we got here, David was here with someone else."

Mary Ellen, who was in on the experiment, played along by pretending to be in shock and disbelief.

Kiley told her David was not in Atlantic City, as Mary Ellen thought, but had just been at the restaurant with an attractive blonde. When Mary Ellen continued to ask whether it was her David, Kiley assured her that not only could she see him in the mirror, but that he fled the restaurant after her text message to him.

Kiley looked very uncomfortable, explaining how they were holding hands and being intimate. She demonstrated how they kissed.

At that point our correspondent, John Quinones, walked in to end Kiley's misery.

A mix of surprise, confusion and relief appeared on Kiley's face as Quinones revealed her unwitting part in our experiment. "I hope it didn't upset you too much," Quinones said. Kiley joked to Mary Ellen that she hated her, unable to suppress her smiles and laughs.

Quinones asked her why she told Mary Ellen. "Because you shouldn't live a lie," Kiley replied.

Of course, Kiley had a little help mustering the courage to come clean. Remember that phone call? We asked Kiley's mom what she told her daughter.

"I said 'Kiley, you have to, you have to tell Mary the truth, you have to'," said Donna Taylor, Kiley's mother. "No. 1, if she finds out that you knew and you never told her, she will never speak to you again and … it's the right thing to do."

It turned out Taylor had her own deeply personal reasons for wanting her daughter to tell the truth. She said her husband had an affair, "and when I found out, I felt, I was humiliated, and I felt like a fool. Everybody knew he was cheating on me except for myself."

Drudging up her hurtful past was a lesson Taylor said she had to give to her daughter. "I would have really appreciated if somebody had come forward right in the beginning and said, 'Donna, this is what's going on, you need to know.'"

If you want to talk to others about infidelity visit www.survivinginfidelity.com.