March 28, 2015 09:05 AM

Lesson to all would-be cheaters: If you’re going to be unfaithful, don’t let Patti Stanger see it.

On Thursday’s Watch What Happens Live, the Millionaire Matchmaker star told a shocked Andy Cohen that she was a firsthand witness to Mario Singer stepping out on his now-estranged wife Ramona Singer.

Now, Stanger has exclusively talked to PEOPLE to fill in the details of the night when she “saw the whole thing go down.” In fact, Stanger says she witnessed what she believes was the beginning of Mario’s long-term affair with twenty-something Kyle “Kasey” Dexter.

When a WWHL caller asked Stanger, 53, if she’d seen any “warning signs” that the Real Housewives of New York star and her husband might be headed for divorce, Stanger said she saw one giant red flag: “I was there that night he cheated.”

“I saw him pick the girl,” she told Cohen, 46.

Stanger told PEOPLE she’d been at another dinner with Ramona’s former RHONY costar (and sometime nemesis) Jill Zarin before they both headed to the party and spotted Ramona, 58, and Mario, 61, engage in a flirt-off.

Very quickly, says Stanger, “I noticed that everybody there was a little too wasted. Everybody seemed a little too flirty – almost overtly.”

“We went to the dessert table,” she says, “and I noticed that Mario was talking to this young lady who was a little bit out of his league. She was very young. In her twenties.”

“Ramona was watching,” adds Stanger. “Ramona was mad.”

Though Stanger says Ramona was also chatting up a younger man, her husband had already made a connection with Dexter.

“You could see he was a target, but that he was excited to be a target. They both went for each other and eyes locked,” said Stanger.

“I just felt creepy about the whole thing. I feel sorry for her.”

She recalls that, later, “When I saw the stories about him cheating on Ramona with someone else, I said, ‘That’s the girl!’ ”

After the initial shocked wore off, Stanger told Cohen, 46, that she “was worried for Ramona. I was really worried for her because I saw shades of … midlife crisis going on, and she doesn’t deserve that.”

Stanger says she reached out to her friend: “I talked to Ramona about it. I told her, ‘Call me when you are ready. I’m taking your side because I don’t think its fair. I said, ‘This happens a lot.’ ”

More emotional fallout from the Singers’ split will be address when The Real Housewives of New York City‘s returns to Bravo on April 7.

Reporting by KC BAKER

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