Wanna make someone feel worse when they've cancer? Here's how:

Talk even when you don't know what to say - Years ago when my friend's wife was diagnosed with leukemia, his buddy didn't know what to say, so he said this: “Man, at least you get to marry another woman...”
Begin your sentences with “At least” - If you want to minimize one's experience with cancer (or anything for that matter) and remind them how much better they have it compared to people in Syria, the words, “at least” is the way to go.
Tell them you understand when you really don't - A patient has taught me I can only guess or imagine what it's like to be in their shoes; to say that I understand when I've never been can be a real put off.
Share about your aunt's cancer story, and your grandma's... - Not to say there isn't a time and place to share cancer experiences, but just because you have a story to tell does not mean the one with the cancer wants to hear it. It helps to ask first.
Expect that things are back to normal post-treatment – Meal delivery is over, hair is growing back, party pics made it on FB, life's back to normal, right? Cancer brings a new normal. While it might not be your job to constantly remind the other of their cancer, just know that this is only the beginning of the journey.

Here's an earlier post on what to do instead.

And if you need help navigating through all this, give me a call!