According to Sydney, they met in November but hooked up earlier this year while he was dating Khloé. The model says that Tristan told her he was single and his baby mama wasn't "his type."
Sydney also alleges that Tristan reached out to her on April 13 -- the day after his daughter with Khloé's 3rd birthday -- when he was in Boston.
In the latest KUWTK teaser, the Boston Celtics player gave no clue that he might have been cheating while FaceTiming Khloé all the way across the country.
The preview shows Khloé talking to Tristan on her iPhone and he's heard saying, "I miss you."
"How's Boston?" a clearly excited Khloé asked her beau, to which Tristan responded, "I really want you guys [Khloé and True] here. I miss you guys."
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He then tugged at her heartstrings by adding, "It's not the same without y'all." Clearly, KUWTK editors are working overtime because while the phone call seemed to go well, there appears to be some indication that trouble was brewing between Tristan and Khloé.
In the next scene, Kris Jenner can be heard telling her daughter, "There's nothing that I would love more than to see you grow old with the dad of your children" as dramatic music plays in the background.
Khloé made headlines when she revealed the duo were getting serious about surrogacy. The idea of giving True a sibling might be put on hold as the couple figures out what is next following the latest cheating scandal.
While the NBA star has kept quiet following the rumors, Khloé seemingly broke her silence last week. She also slipped off her enormous diamond ring after the allegations which says it all.