Updated: Helen McCrory has died aged 52

Actress Helen McCrory, known as Narcissa Malfoy in Harry Potter, has died at the age of 52. Her husband Damian Lewis confirmed the news on Twitter. He explained that she died following ‘an heroic battle with cancer’. Daniel was among the first to pay tribute. A journalist for Metro.co.uk shared Daniel's statement released by his publicist on Twitter.

Update: 17th April 2021. The statement is now also online on his official website.

"I truly couldn't believe this news when I heard it. Helen was one of the most incredible actors I got to work with on the Harry Potter films. She was effortlessly commanding and captivating as an actor, as well as being kind, generous, and incredibly good fun as a person. 
Also, as it happens,  she's one of the absolute coolest people I've ever met, in that I can't ever imagine her being flustered or nervous, she always seemed totally assured of who she was and what she was doing.  She was a supreme talent and a wonderful person and will be very greatly missed."

-Dan

Back in 2011 Helen called Daniel the "ambassador" of the Harry Potter series (Parade magazine).

"It's sad that it's coming to an end. I enjoyed it so much. I felt very welcomed as soon as I came on set by the crew and the cast alike - particularly by Daniel Radcliffe. You feel he's the ambassador to the embassy that you're joining. He really welcomes you on and introduces you to everybody and is a real gentleman,"

source: metro.co.uk