Am truly sorry your friend is in this club but it's a well-trodden path, one many of us have been down, and has amazing survival rates. Research into breast cancer, not to mention treatment, particularly surgery has come a long long way. If she is having chemo first, then surgery, that is because they want to see how the tumour responds/if it can be shrunk. This in itself, along with other factors will determine whether it will be breast-conversing surgey (lumpectomy), a mastectomy, or a double mastectomy.
It is really thoughtful of you to want to do something like this. But, and forgive me for being so direct but you're getting ahead of yourself. Additionally, this is something covered by the hospital and/or Macmillan (who will refer). Regrettably, am on holiday at the moment so unable to take a look at the brand, but IIRC, it is Amoena). It is standard practice to wake up in a bra after a mastectomy. (I say this from experience, have had two mastectomies a year apart). Your friend will be advised/fitted by someone who knows what they are doing, so for now, you really don't need to do anything. However, and at the same time, am not sure what the procedure is for a lumpectomy as my last one was over a decade ago. In those instances - I have had five - I was taped up and advised beforehand to buy a soft comfortable bra, no wires. I went to Bravissimo for this but these are pretty easy to buy. Many opt for crop tops like Sloggi, something that can be stepped into and pulled up rather than pulled down. What is more important IMO is ensuring that everything is within easy reach as they won't be able to raise their arm(s). Ditto clothing that is easy to put on. Ergo, front fastening items are important for the first few weeks, longer if she's having a mastectomy. If she does need to have a mastectomy, a mastectomy pillow from this seller would be brilliant, I still have mine. The seller is a wealth of information and happy to answer questions.
An even better purchase would be this book. Tell your friend to only read what is applicable to her as and when she needs the information. There's no point getting ahead of herself not least because cancer plans are ever changing, and there's no point worrying/thinking about treatment she likely won't need.
To return to bras, it is also worth considering if your friend will be having any reconstruction? Also, will she be having a skin saving mastectomy, or will be an aesthetic flat closure? (I had this). If opting for reconstruction, some have it immediately, others do not but have an expander put in place. Also, will she be having radiotherapy because again, that will inform the kind of bra she will need. Ultimately, for the first few months, the bra(s) the NHS provide will be best. They're not the prettiest but they're the most functional for the purpose. They will hold a prosthetic (also provided by the NHS) should she choose to wear one. (Again, I did not wear these, although for the first few months after I was able to sleep on my front again, I did get a knitted knocker to even myself out when I slept!)
As mentioned, I have had a double mastectomy, albeit in two stages. It took me eighteen months or so until I started thinking about buying new lingerie. So, to return to your original question, if you still really want to buy her some pretty lingerie, wait until you know exactly what they end result will be. And if you still want to proceed, I can recommend Love Rose Lingerie - the woman behind it has had breast cancer and really lovely. Instead, why don't you buy her a pretty camisole to sleep in/wear over her recovery bra? I know this was something I loved getting, as were some vests with the secret support panels.
Wishing your friend all the best 💐
As an aside, I know you mean well but please, not all of us like the term warrior. Cancer isn't a battle to be fought, and to a degree, it implies those who succumb haven't fought hard enough. This is never the case at all.