The celebrities, who are living together in Blac Chyna's gated Los Angeles community, were spotted driving to his mom Kris Jenner's Calabasas home.
Rob, 28, was behind the wheel while the 27-year-old sat in the passenger seat. The outing marks a rare public appearance for the reclusive Keeping Up with the Kardashians star.
Rob, who was recently diagnosed with diabetes, found an escape from his family "nagging him about his health" and has "ignored their attempts to contact him," the source added.
The star was previously living with sister Khloé Kardashian, who sources say doesn't approve of the relationship.
"Khloé feels very betrayed," another insider told PEOPLE.
Black Chyna has a long history with the family and was once close friends with Kim Kardashian West. Things seemed to go south, however, when Blac Chyna's ex-fiancé and father of her child, rapper Tyga, began dating Kylie Jenner.