Be there as often as you can. I remember it getting hard when ppl kept asking "how are you feeling?". Bc if I was honest I would have said I feel like crap, I'm scared, I feel sick, I'm afraid of dying, etc. So, get in the habit of saying: I hope you're feeling well today, or I was thinking of you today. Also, don't say "let me know if there's something I can do" bc it puts the burden back on her and it's so hard to ask for help. Instead, ask when her appointments are and plan to go with her (if she has no one else that can go), stop by (call or text first) with a meal when she's sick from chemo and clean up a little while you're there. Bring funny movies or books ('the sh*t my dad says' is hilarious--someone gave it to me), bring gossip, distractions are good. Try not to probe by asking a ton of questions all the time, but let her know you're always there to listen. She'll start to open up when she wants. If she's sad, let her be. Be comforting but don't give advice. (like empathize and say you know it must be hard and scary, but don't say things like, look for the silver linking, or try to be positive...some days, she'll just be sad and angry will need a shoulder to cry on)
When she's feeling well, keep her busy! If you aren't always free, create a calendar for friends/colleagues that can cook, visit, take her out, etc.
If she plans on wearing a wig, offer to go with her to pick it out before her hair falls out. Then, when it starts to fall out, offer to shave it (my friend gave me a Mohawk).
When her treatments are over, months from now, keep checking in...that's a tough time emotionally, even when hair starts to grow back. Breast cancer is life changing and we still think about it even post treatment.
Of course, you can't do it all, but get your friends together to help with all of this.
I've truly seen who my true friends are with how they've dealt with my diagnosis. I'm young(32), and I've read and agree that breast cancer is lonely for young women bc most of our peers have no idea what it's like. If your friend is young, help her check out programs for young women with BC