You not calling, as a friend, can actually compound the grief and loss they are feeling. Just pick up the phone, even if you get it wrong, just have a conversation and do your best. Your friend with cancer is still the same person they were before.
In the beginning, it was difficult to let myself rest, even with the ultimate doctor’s note. I felt like I still had to push past my (extremely limited) capabilities. I had to practice slowing down and allowing my frailties to become visible even when I had the choice to hide them.