How Do I Console My Best Friend Whose Mom Has Been Diagnosed With Cancer?

My mom told me that she was told that my best friend's mom has cancer and they dont know how serious it is. My best friend is obviously upset and my mom said i shoudl speak to her. My friend hasn't told me this news by the way, so I dont know the best way to approach her over the phone to console her. What do I say or ask? I dont want to say anything insensitive or stupid and what advice should i give to my friend?