Two years ago the actresses reunited professionally, on Bertinelii's series Hot in Cleveland.
Bertinelli said in a statement Friday: "My heart is breaking. Bonnie has always been one of the most important women in my life and was a second mother to me."
"The years on One Day at a Time were some of the happiest of my life, and along with Pat [Harrington Jr.] and Mackenzie [Phillips] we were a family in every way.
"She taught me how to navigate this business and life itself with grace and humor, and to always be true to yourself. I will miss her terribly."
In her own statement, Phillips adds, "I am so saddened by the loss of our dear friend. She was just full of light and love. Bonnie will be very much missed by all the people she touched ... We all loved her very much.”