According to multiple sources, Doug, a 55-year-old businessman, entered an inpatient rehabilitation center for addiction after the split, Us Weeklyreported.
"Christina supports Doug's decision to seek treatment and remains in contact to support his recovery," a source told Us.
In Touch Weeklyreported that Christina, who stars on Flip or Flop, discovered Doug was meeting up with an ex-girlfriend, Tracy Diaz, behind her back.
"Christina went to put an alarm on Doug's cell phone to wake him up and she saw text messages [between Doug and Tracy]," an insider told In Touch.
Christina also reportedly found out that Doug was doing drugs again before she ended their relationship.
Doug had been arrested for possession of methamphetamine in July 2011 but subsequently completed a diversion program in 2013. Doug's troubled past also reportedly includes a thrice-violated restraining order taken out against him by his ex Nicole Doucette, the mother of his 10-year-old twin daughters.
"Christina encouraged him to go back to rehab. Christina is the type of person who sees the best side of others, and that's how she was with Doug," another insider told In Touch. "Caring about someone does not save someone and Christina knows that. She just wanted to help him."
The 34-year-old HGTV star began dating Doug in May although they've known each other for about a decade.
Christina and Doug first dated years ago before she married her now-estranged husband andFlip or Flop co-star Tarek El Moussa.
Christina and Tarek, 36 -- who share daughter Taylor, 7, and son Brayden, 2 -- publicly announced their split in December 2016 after seven years of marriage. The couple, however, had quietly separated in May of that year following a blowout fight.
Tarek filed for divorce from the blonde beauty in January 2017, and she legally responded in August. Despite their high-profile divorce, the reality TV stars continue to co-parent their children amicably and work together for appearances or joint real-estate and home-renovating projects.
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Flip or Flop's seventh season will premiere in December 2017 with 20 new episodes, which is more than a traditional full-season order, on HGTV.