@fack check to see if they have a special 'top surgery' pillow, or if there's post-operation things that would be 'nice to have' that they haven't already gotten.

There's some drainage pocket hoodie garment that folks often like. Maybe not this one exactly, but something like it. Your friend probably already knows this/has a plan. https://www.etsy.com/listing/542496412/post-mastectomy-hoodie-with-surgical

Otherwise, I'd ask them and just focus on just bein' there. Visit them and help out with a meal, and then clean up afterwards. Surgical recovery is boring and tiring, and it's likely that they might feel shy about having trouble/not supposed to do housework etc.

I loaded up an old e reader with books I loved for my partner, and that seemed to work really well. Another friend stocked up on coloring books. Podcast/show recommendations might also be good, but I'd keep to pretty light things that you can still follow okay if you doze off for a few minutes.

You might also get them a gift card for your favorite place to get shirts/dress shirts.

Generally, I'd send them a text and say something like "Congrats on your surgery date! Can I help with anything?"