The country singer took her own life at her Arkansas home earlier this month, 36 days after her partner David Wilson passed away in the same place, and Bob Forrest - who worked with the late star on 'Celebrity Rehab' in 2009 - was surprised by her actions.
He said: "I never thought Mindy would take her own life. I always thought if she died it would be at the hands of another because of her past physically abusive relationships with men.
"Mindy had lost custody of her kids before and didn't kill herself then. I absolutely believe that David Wilson's suicide a month ago pushed her over the edge and caused her to take her own life. This was someone that Mindy had a history and a child with, and they seemed to be working through their issues. It's just so sad that she took her own life."
Before shooting herself, Mindy - who had five-year-old son Zander with ex-boyfriend Billy McKnight, and 10-month-old Zayne with David - killed her pet dog and Bob believes the action demonstrated she wasn't in her "right mind" at the time of her death.
He told RadarOnline.com: "It was rage and anger that caused her to do that.
"Mindy obviously wasn't in her right mind when she picked up the gun on that night. She had underlying psychiatric issues and an addiction to alcohol and prescription drugs - and it was the perfect storm of events colliding all at once."