Khloé Kardashian 'Upset' Over Tristan Thompson's Cheating, 3rd Child: 'They Were Still Together'

A source tells PEOPLE that Khloé Kardashian "knows about the baby" that Tristan Thompson has allegedly fathered with Texas personal trainer Maralee Nichols.

According to court documents obtained by PEOPLE, Nichols is suing Thompson for child support and other pregnancy-related fees after she said they conceived a child together in March. The Sacramento Kings player confessed to having sex with the personal trainer on at least one occasion, per the documents.

"Khloé was upset to find out that he cheated again," says the source. "She just can't believe that he is going to be a dad again."

The insider confirms that Kardashian, 37, and Thompson, 30, were together in March when Nichols claims she was involved with the NBA star. "It's just a sad situation to her," they add.

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Thompson is already dad to daughter True, 3, whom he shares with Khloé Kardashian, plus son Prince, 4, whom he shares with ex-girlfriend Jordan Craig.

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George Pimentel/Getty Images; Khloe Kardashian/Instagram Tristan Thompson and Khloé Kardashian

The source tells PEOPLE that Kardashian had friends encouraging her "over and over again" not to take Thompson back, but ultimately remained by Thompson's side "because she always believed that he would change" and wanted to keep the family intact for True.

"She is just a really great person and wants to see the positive in people," they say.

The insider added that the couple was "doing great" while quarantined together during the COVID-19 lockdown, but that Kardashian "knew things would change" as soon as he returned to the NBA. "She was just hopeful that he would stay faithful and loyal to her," says the source.

"Khloe's family always tried to support her," they add. "They always treated Tristan well even after he broke Khloe's heart multiple times."

Tristan Thompson Instagram Tristan Thompson and Khloé Kardashian with daughter True

RELATED: Tristan Thompson Shares Sweet Photos of His Kids True, 3, and Prince, 4, Together: 'My Everything'

Thompson and Kardashian have been on and off for several years, and most recently split in June. At one point, the former couple was exploring the idea of having another child together, but their plans for a surrogate fell through last summer. A source previously told PEOPLE that the exes are continuing to "keep things friendly" for True's sake.

In October, the Sacramento Kings player shared rare photos of his two children hanging out together at an arcade. The first snap shows the siblings standing side-by-side with big smiles while another shows True looking fondly at her big brother.

"Let me love you a little more, before you are not so little anymore #MyEverything❤️❤️," Thompson captioned the sweet Instagram post.

Kardashian commented on the photo, writing, "They are so beautiful and sweet!!!! So so blessed!!! What angels! ❤️."

The NBA player typically shares snaps while spending time with his kids individually. Thompson previously gave fans a glimpse of his kids together back in 2018 after True was born, posting a family portrait on Instagram for the first time.