Bethenny’s Betrayal! Frankel Befriends Ramona’s Nemesis Kasey Dexter

Bethenny Frankel and Ramona Singer are at war this season on The Real Housewives of New York — and now RadarOnline.com can exclusively reveal that there could be a secret reason behind their feud!

The Skinnygirl mogul resorted to staying at her friends' Hamptons homes before she bought her own pad, and befriended Ramona's husband Mario's then-mistress while they stayed at the same house!

“Kasey Dexter and Mario met at their friend’s home in the Hamptons in 2013,” an insider told RadarOnline.com. “She had a bruise on her butt from getting hit with a tennis ball and he came up to her and was flirting with her, talking about the injury. Ramona was right there and witnessed the whole thing. Mario asked for her number and the next day she sent him pictures. They were on their friend’s yacht and Mario almost had a heart attack!”

“Bethenny was staying at the same house as Kasey that summer,” the insider added. “They used to be friends!”

“Bethenny knew everything that was going on!” the insider concluded. “Maybe that’s why Bethenny and Ramona deep down don’t like each other!”

RadarOnline.com previously reported that Ramona filed for divorce after she caught Mario with Kasey at their Hamptons house. She later revealed that their separation led her to contemplate suicide.

Mario, 64, continued his relationship with Kasey, 34, and they moved to Florida together two years ago. Now, they are considering tying the knot soon.

Meanwhile, Bethenny, 46, and Ramona, 60, have been at each other’s throats since Ramona revealed the Skinnygirl’s “soft porn” past on The Real Housewives of New York and asked if she told her seven-year-old daughter Brynn about it.

Stay with RadarOnline.com for more.