A friend whose brother just passed posted this on FB and I think it's worth sharing.
Asking "what can I do?" burdens the already exhausted person with coming up with something to be done and assumes s/he knows what needs to be done.
I learned a lot about this when my FiL broke his neck last November and I spent the next four days being a barrier to this for my MiL. While he was in surgery I made Mr GV take a nap and then dictated texts for his mom and told her who to call and when and fielded calls from FiL's sister (a well-meaning nurse who was very worried). I didn't know what she needed done and I didn't want to overdo it, but she looked so uncharacteristically bewildered I started doing things.
People call to express support and it ends up being stressful. One of the best things was the old friend of theirs who dropped off two rotisserie chickens and a giant slab of ham from Costco and left with barely a word. Others sent texts saying something along the lines of "I heard what happened and I'm so sorry. I'll get in touch in a couple days. You're in my thoughts."