I had cancer when I was younger, and the best place to start is to ask yourself: "Do I need to say anything at all?"
Unless you're a close friend (with whom the patient is completely at ease), then you're putting them in a position where they feel like they have to act strong, smile, and thank you for your concern. (For my situation, I got dozens of phone calls from people I hadn't heard from much in years. I had more than enough on my mind than to appease their desire to show their concern.)
Your concern is definitely appreciated, but you need to choose your moments.
As for what to say? I guess that differs culturally and your relationship. I'm Australian, and I was totally okay with a simple "ah, shit mate, that sucks" over a "if you need ANYTHING, call me* (*but don't actually call me)" - I know I can call for help, and dozens of people can help, but all I really wanted was to be treated like a normal human. For instance, I was bald from the chemo and got a multi-colour clown wig as a joke present, which made me smile so much. It was a sense of "everything is normal despite being in the midst of chaos." I could count on my friends to distract me from the horrible reality of the situation when I was stressed/sad/confused.
And that is your job. Be a sincere option for distraction and advice when you're called upon. Trust them to ask for help when they need it..and they're much more likely to ask for it if they know you won't be overbearing/over-worried/judgemental/etc.
I don't know how well this advice applies to depression, but I think it's a good framework for being a good friend through most hardship.
(Sidebar: I'm more than happy to be a sounding board for approaching any cancer related issues you've got. Email's in my profile. I've heard all kinds of things, so don't think any question or issue is too simple or stupid!)