Now one of Hollywood's longest-standing bonds is bringing comfort and support to Harper as she faces terminal brain cancer.
"I'm absolutely devastated by this news," says Moore, 76, who herself underwent surgery in 2011 for a benign tumor in the protective membrane around the brain – the same spot where Harper's malignant cancer cells are located.
"Valerie has given so much joy, laughter and love to the world," says Moore. "I join her fans and send much love and positive thoughts to her and her family during this difficult time."
One day before going public with her diagnosis in PEOPLE, Harper called her costar from the show Mary Tyler Moore.
After small talk, "She said, 'What's up?'" recalls Harper, 73. "I said, 'It's incurable cancer.' She said, 'Val, I'm so sorry.'"
Both showbiz veterans have endured their share of personal challenges through the years, including Moore's public battle with diabetes and the untimely deaths of her sister and son.
"In one of her books she said, 'At every funeral of my family, Val was at my side," says Harper. "It's true. I love her."
And Moore continues to share support and advice with her dear friend.
Before ending their phone call, "Mary said, 'One day at a time,'" says Harper. "That's how we've got to live our lives. That's the best we can do."
Reporting by JULIE JORDAN