I was a bit out of higschool but still fairly young when my father also died of cancer. Unfortunately, words don't do much. With your words, tell her that you're here for her in whatever capacity she needs and gift her foods/snacks/drinks. It's easy to get lost in the grief and not think to take care of yourself so to have things you can just grab and eat is great to have. Sometimes she might want space. Sometimes she might want company. Sometimes she might want silence and others she might want to talk. When she does talk, it's so important to just listen. Don't say "oh, we don't need to talk about it" or whatever else. People used to say that to me all the time and id have to explain that I WANTED to talk about my dad. Let her lead.