Walsh, 31, struck up a strong bond with the brunette beauty after they joined the chart-topping band Girls Aloud in 2002 and their friendship has been growing ever since they met, reported Contactmusic.
"I'm very protective of Cheryl and if she hadn't wanted me to address her split from Ashley (in my new autobiography 'A Whole Lot of History') I wouldn't have done. But she was quite happy for me to speak about it from my side. In fact, she encouraged it.
"All I could really do was be there for her, physically, 24 hours a day. I'd been through it with my sister, so I knew what someone's mental state is like when they've had that sort of shock and I didn't want her to be on her own," Walsh said.
"Cheryl's in a great place now. She lives closer to me when she's here in the UK and that's great... We're friends for life and often joke about how we'll be reminiscing to our kids and grandkids in year to come about how were in a band and how we climbed a mountain together," she added.