It doesn’t sound as if you would, but for the avoidance of doubt don’t send uplifting memes, quotes etc and don’t anthropomorphise cancer “fuck you cancer!”, “you’re going to kick cancer’s butt!” etc etc.
Don’t say “I know you’re going to be fine” or, as others have said, suggest crackpot cures.
Don’t say “I know lots of people with that”, which a couple of friends have told me, which made me feel a bit dismissed as if it was no big deal if they’ve already seen others go through it.
When she tells you about her treatment plan (if she does), pay attention to which day of the week her chemo will be on and how many sessions she’s having. I really disliked telling the same people over and over what the plan was/where I was in the schedule and it being obvious from their messages that they hadn’t really listened when I had taken them through it all. (They had asked for all the details and I was happy to tell them for their information, but I did expect the quid pro quo to be that they’d do something useful with it!)
If you know which days she’ll be having chemo, a regular “hope it goes well today/tomorrow” was sometime found really thoughtful from those who were on the ball.
Ask if you can come along for a chemo session. I was really touched that people wanted to, and did, come along.
Don’t say “you must be nearly finished!” even if she’s a day away from finishing. Until you get to the finishing line, it’s always a long way away for the patient, and treatment might be called off on the day, and of course for some their treatment is ongoing for life.
Don’t take flowers - IMO they’re a bit funereal and a hassle to get rid of when they die. Skin care or a book or makeup or something would be more welcome for me, personally.
Do ask her whether she wants to talk about how she’s feeling or something else. Often I get a bit fed up talking about me and it’s a relief to hear about something else, however mundane you might think it is in comparison.
It’s great that you are asking for tips on this and I hope your friend is doing ok. Do send her along to the Cancer Support thread on here if she’s interested!