The woman in question, Eleonora Srugo, a New York City-based real estate agent, "has been talking to her friends about sneaking around with Dale," according to a source.
"[Clare] has seen proof" that when she couldn't be with Dale in New York City, he was reportedly spending time with Eleonora.
Dale reportedly claimed his relationship with Eleonora was strictly platonic and business-related, but the source told E! News that Clare "never trusted the friendship and thought it was shady."
"Clare has always been skeptical" of Dale and Eleonora's alleged friendship, which reportedly dates back to at least late 2019.
Dale was once photographed having dinner with Eleonora and other friends, and he has commented on her Instagram account several times since 2019. Their arguably flirty interactions reportedly continued in the months preceding Dale's The Bachelorette appearance.
"Dale would always reassure Clare that it was nothing serious," said the source.
"[But] multiple people have told Clare that Dale was parading around NYC with this girl."
The source explained that "Clare would confront him about his relationship and tried to question him about it, but he would always deny it."
Dale's questionable relationship with Eleonora was reportedly "a root cause of a lot of their arguments" that resulted in Dale and Clare parting ways and ending their engagement.
Dale announced his split from Clare on Tuesday, January 19, saying the pair had arrived at "the healthiest decision" for the "both" of them, but then Clare revealed two days later she had been blindsided by Dale's public statement.
Clare wrote on Instagram on Thursday, "I was made aware of a 'mutual' statement at the same time you all were, so I've needed some time to really digest this."
Multiple sources described Dale as a "fame-seeker," according to E! News, and the source said, "It is obvious [to Clare] now. Things are making sense to her now. She didn't want to believe it at first but is now heartbroken."
Another source added of Clare, "She feels [Dale] knew the truth was going to come out about his shady friendship. She feels he wanted to break up quickly because the truth was eventually going to come out."
However, a source close to Dale has denied Dale and Eleonora were having an affair.
"There is zero truth to the romance rumors," the Dale insider told E! News. "Dale was faithful to Clare throughout their entire relationship. He is committed to staying on good terms with Clare and has nothing but love and respect for her."
The Dale insider claimed the pair work out at the same gym and Eleonora "was helping Dale find a bigger apartment for when Clare would eventually move to New York."
"He has known the girl for way longer than he has known Clare," the Dale insider alleged, "so of course they would be in touch... This is an innocent friendship."
An Eleonora friend explained Dale and the real estate agent had simply developed an "intimate relationship" just like many brokers and their clients.
"They become their social workers, they know the client's financials, they become their dog sitters in some cases. They can become like family," the close pal told the website, adding that Eleonora is a professional.
"In this case, Eleonora and Dale are close friends, but that is all... She definitely wouldn't sleep with someone's fiance."
In addition to cheating speculation, it's been widely reported Dale "broke it off" with Clare because he wants to continue living in New York City while Clare feels the need to stay in Sacramento, CA, and tend to her mother, who battles Alzheimer's disease.
"[Dale] doesn't want to give up the party life. He loves the NYC scene," the source said. "His partying would be a problem in their relationship and would cause many fights."
In addition, the source noted of Clare, "She just wants to get married, settle down and have children. They were on complete different pages."
When a reporter tracked Dale down outside of his New York City apartment, Dale reportedly explained of the breakup, "Honestly, we've just got a lot going on in our lives right now."
When asked whether he and Clare will remain friends, Dale responded, "Clare and I will be cool."
But Clare said in her Thursday Instagram statement she is "crushed" by the breakup given her intentions with the relationship had "always been very clear."
"This was not what I expected or hoped for and am still trying to process this," Clare wrote in her post.
"2020 was one hell of a year, with COVID, battling severe anxiety post-show, balancing a public new relationship, all while slowly losing my mother. It hasn't been ideal circumstances, but that is life right."
Clare, however, said she has "been looking forward to the light at the end of the tunnel."
"Our relationship was not perfect," she continued of Dale, "but I can say that I was genuinely invested with all of my heart. I may not have all the answers, but I do know this -- I will continue to show up, stand by my word, and be committed to love."
Before Dale announced his split from Clare, seemingly without Clare's knowledge, reports swirled the pair had been fighting all the time and so they decided to take some time apart to figure out their relationship and where it was headed.
Clare and Dale last posted photos of each other on Instagram on January 9, when the couple enjoyed a beach day in La Jolla together and their relationship seemed to be in good standing.
Clare was convinced she had met her future husband when Dale stepped out of a limo on Night 1 of The Bachelorette's sixteenth season and took her breath away.
It took less than two weeks for Clare to determine Dale was the guy she wanted to be with forever, and so she dumped all of her other The Bachelorette bachelors to pursue that one relationship.
Clare's journey on the show therefore ended prematurely with a steamy overnight date with Dale that led to Dale getting down on one knee and proposing marriage seemingly the next day.
When Chris asked what was next for the couple, Clare immediately shouted, "Babies!"
But Dale noted he'd like to get married before welcoming children.
"I've never been afraid of commitment," Dale told People in a joint interview with Clare last year.
"And when I met Clare, I knew I was all in. She lights a fire in me that I was missing for so long."
Dale recently claimed he had turned down the opportunity to compete on The Bachelorette multiple times after being nominated for the show but changed his mind once Clare was announced as the show's sixteenth-season star in March 2020.
Prior to getting involved with Dale, Clare had been engaged to The Bachelor Winter Games bachelor Benoit Beausejour-Savard.