"She's incredible. When they say a warrior, she is a warrior. She lives every day and she's fighting," the Buffy The Vampire star, 46, told Access Hollywood.
"There are ups and downs and there are days that are harder than others and there are some days that are easy, but she's doing great.
Doherty, 52, who was first diagnosed with breast cancer in 2015, revealed in June that the disease had spread to her brain.
The Beverley Hills 90210 and Charmed star later shared a video of herself lying in a hospital bed before having surgery.
"I had a tumor in my head they wanted to remove and also biopsy," she wrote in the accompanying caption.
Adding: "I am clearly trying to be brave but I am petrified. The fear was overwhelming to me.
"Scared of all possible bad outcomes, worried about leaving my mom and how that would impact her. Worried that I would come out of surgery not me anymore. This is what cancer can look like."