ET's Brendon Geoffrion caught up with the 45-year-old actress at the American Cancer Society's Inaugural Giants of Science Gala at the Four Seasons Hotel in Beverly Hills, California, on Saturday, where she reacted to the news that Buble's son, Noah, was recently diagnosed with cancer.
"I can't even begin to touch that one, to be honest, because it's a 3-year-old," she said. "I would imagine your child having cancer is devastating as a parent."
Doherty -- who was diagnosed with breast cancer in February 2015 and revealed she was "nearly finished" with her chemotherapy treatments this past September -- said the best thing Buble and his family can do is "stay positive."
"Just trust in your doctors. Make sure you have amazing doctors, a great team, and know that organizations like the American Cancer Society [and] Stand Up to Cancer, they are doing so much good for cancer," she noted. "And [there is] so much research that we have much better chances now."
"Stay positive," she added. "That positivity, that outlook on life that you can get through something, it actually does get you through."
Buble revealed on Friday that Noah has been diagnosed with cancer, and he and his wife, Luisana Lopilato, would be taking time off to care for their son.
"We are devastated about the recent cancer diagnosis of our oldest son Noah who is currently undergoing treatment in the U.S.," the 41-year-old singer said in a statement on his Facebook page. "We have always been very vocal about the importance of family and the love we have for our children."
"At this difficult time, we ask only for your prayers and respect for our privacy," the statement continued. "We have a long journey in front of us and hope that with the support of family, friends and fans around the world, we will win this battle, God willing."
ET recently spoke with Buble about how he's changed since becoming a father to Noah and 9-month-old son, Elias.
"I love them more than I'll ever love anything or anyone in the whole universe," he said. "You're scared all the time, because you love them so much that you can't ever, ever think of a world without them. You try not to even think about it."