Message them every other day just saying 'hope you're feeling ok today'
With all due respect to Blue (who I’m sure is a wonderful friend) if anyone had done the above I’d have been driven round the bend.
Like most things, this stuff is personal - there’s no one size fits all.
I didn’t want lots of support. I’d have been completely non plussed if someone turned up with food or gifts.
My DH didn’t need to “look after me”. I was fine.
What I did need very occasionally was a friend to listen to my worries or nerves.
My advice would be follow your friend’s lead. Don’t push your way in and make this about proving what an awesome friend you are (happens more than you’d think).
Be there if they need you. Back off if they don’t.
Talk about other things not just the treatment.
Don’t ask for updates.
Don’t go overboard if the treatment is successful, there’s still a long way to go.
I hope things go well for your friend. 