"I don't really know what to say."
It's OK for you to be honest about how you're feeling. We don't expect you to be an expert in dealing with this really tough situation.
"I'm sorry you're going through this."
Sometimes a simple acknowledgement that things are a bit crap right now really helps.
"Do you need a lift home from chemo?"
Specific offers of help are much better than general "I'm here if you need me" type statements. We'll actually take you up on them, and they will genuinely make a difference.
"Have you seen...?"
Movie and telly recommendations are invaluable for days when we can't get off the couch and feel like we've exhausted Netflix.