"Rob is such a great guy," Khloé, 31, told PEOPLE in October. "He's charming, charismatic, everything. That's what's so infuriating. I'm like, 'Where is your will to live life? Let's be around people!' "
So it hurt the reality star when the person who seems to have given the push he needed to kickstart a healthy lifestyle turned out to be a Kardashian enemy: Blac Chyna, who was once engaged to and shares a son with their sister Kylie Jenner's boyfriend Tyga.
In this week's issue of PEOPLE, an insider says Khloé felt "betrayed" by the blossoming romance.
Alexander Tamargo / Getty Images; Jeff Vespa / Getty Images for The Hollywood Reporter
For more on Rob Kardashian's relationship with Blac Chyna, her arrest and how the Kardashian-Jenner family is reacting, pick up this week's issue of PEOPLE, on newsstands Friday
As for Khloé, "She was frustrated by his lack of respect," the source says.