The “rule” is – listening is the language of love. Cancer is a tough diagnosis, so your friend needs some compassion. So ask, how is it going – you can do this in person, you can do this thru an email, you can do this in a card. But, only ask, if you are willing to listen. No one is required to care and no one is required to listen. But, if you do want to help your friend – listen to what they have to say and try to understand them. It’s not about you, it’s about your friend.
There are some websites that offer insights into what you might say to your friend: http://www.cancer.org/treatment/understandingyourdiagnosis/talkingaboutcancer/whensomeoneyouknowhascancer/when-somebody-you-know-has-cancer-ways-to-respond
Some things that didn’t work very well for me – because they didn’t know what to say. Some folks told me all the details about every person that they have known that has had cancer, including some cases of excruciating death. This wasn’t too helpful to me – it mostly just told me that cancer is a tough diagnosis – and, I already knew that. And, cancer is a variable illness – each person’s illness, treatment, and impacts are fairly unique – so your facts may not be applicable at all.
Many friends were terrific – they asked how I was doing and they listened, and then a month later, they asked again. They let me talk about my illness, and they let me not talk about my illness. I am very grateful for their encouragement.
Each of your friends will have different challenges with cancer – they will benefit from your support – and I hope you will find the strength in your heart to be a good friend. It makes a difference.