Earlier this week, Munn was photographed leaving the same pad Tendler, 36, and a friend were said to have been packing the artist's stuff in, Page Six reported.
The 40-year-old was "ready to pounce," an OK! insider previously spilled. "She and John had exchanged texts while he was in rehab, and things obviously turned romantic after he got out."
Although friends are happy that Mulaney is doing better, they think "he broke Anna's heart," the insider says. "People definitely have mixed feelings about his new relationship!"
Tendler reportedly entered rehab herself around the same time as her estranged husband, for emotional reasons and eating disorders. Sources have also claimed that she suffered due to alleged infidelity on his part.
"I am heartbroken that John has decided to end our marriage. I wish him support and success as he continues his recovery," she said in a statement following the split. The former couple tied the knot in 2014.
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The former Saturday Night Live writer checked into rehab for 60 days in late December for alcohol and cocaine issues — with Munn being one of the many famous faces to offer support.
"Sending SO MUCH love and support to John Mulaney. You got this. ❤️⚡️," the Magic Mike star tweeted at the time.
Munn previously gushed that she and Mulaney met at a wedding, where she asked him and Tendler to hang out.
"At first, it was cool and then I kept going up to him at the wedding like, 'So you having fun?'" she told HuffPost Live in 2015. "I was just so obsessed with hanging out with him and talking with him."
According to Us Weekly, a source spilled that a "smitten" Munn believes Mulaney is "her dream guy": "Olivia and John are going strong. She has been sneaking into some of his shows and loves them."
"John thinks Olivia is really smart and she makes him laugh too," the source continued. "They both have a very dry sense of humor."