Shane Carey, Broussard's fiance, is now raising their baby Margot as a single father.
Broussard's body was found strangled inside a car trunk at her friend's home in northwest Harris County just days before Christmas. Margot was found alive inside the house.
Megan Fieramusca is charged with kidnapping and tampering in the case.
In an exclusive interview with Good Morning America, Carey said he's still in shock.
"She was the best mom, best person you could ever meet. She had like 25 best friends because everyone wanted to be her best friend. I don't understand what happened, still," Carey said.
Carey told GMA he never would have suspected Fieramusca because they were so close.
"Heidi loved Megan. I mean they had a great relationship. I went over in my head about 1,000 times I should have been there. I should have noticed something," he continued.
Fieramusca was present when Heidi gave birth to Margot and a son, who is 6 years old. She also took part in the search for Broussard.
"She was a friend, you know? Like, you would not suspect this at all," Carey said.
Police say Fieramusca deceived them by allegedly faking her own pregnancy. Carey recalled hearing Fieramusca was pregnant at the same time as Broussard, saying they both had the same due date.
According to authorities, Fieramusca, 33, plotted to steal Margot and pass her off as her own.
Fieramusca was arrested on Dec. 20.
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