Ramona Singer's soon-to-be ex-husband is back with his mistress.

Ramona Singer Doesn't Blame Kasey Dexter For Her Divorce

On Monday, Mario Singer, who split from The Real Housewives of New York City star one year ago, brought Kasey Dexter as his date to the 25A cover party in honor of Geraldo Rivera.

"When they came in, people were shocked he was with her," a guest at the party told Radar Online on Tuesday. "They couldn't believe they were still together."

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While Singer did not attend the New York City event, her co-star and close friend, Sonja Morgan, was in attendance, as was her former co-star Jill Zarin, and her husband, Bobby.

According to the report, Dexter has just recently returned to New York City, and upon arrival, she shared a post about the Big Apple -- and about her "favorite person," Mario.

"They had their arms around each other," the onlooker added. "The were very affectionate."

During an interview last August, Singer revealed why she finally pulled the plug on her relationship months after she caught her husband with Dexter.

"We were working very hard on our relationship and making great progress," she told Us Weekly. "It was really working well, and then I got the feeling that he started seeing [Kasey Dexter] again, [but] he wouldn't admit to it. He said everything was great with us. Then the next morning I pushed him on it and he admitted he [had seen her], and I just said, 'I can't do this anymore. Goodbye.'"

Tags: mario singer ramona singer Bravo Reality TV Kasey Dexter The Real Housewives of New York City