This is a very lovely idea and they will appreciate the thoughtfulness of anything that you give to them. Something to keep in mind is that not everyone reacts to their cancer treatment the same. I have met some who were able to breeze right through with just some exhaustion and mild nausea. My treatment included radiation and chemotherapy that hit me very hard. The nausea was so bad that I couldn't watch anything with a lot of action and I couldn't even read without getting sick. Going into treatment I thought that I would be able to color my way through it, but I couldn't even do that. I could listen to music/movies, look at pictures, read short paragraphs, watch funny cat videos, very slowly put together puzzles, eat, talk, and plan for a better day. Most people will be somewhere in the middle of the two I have described here.
With that caveat, your time is the thing that they would probably value most. Another budget idea is help cleaning the house, or doing some laundry, or making a few meals, bringing them their favorite dessert, and even just spending time with them. If they are able to move around and not suffering from things like tinnitus or motion sickness, then they might appreciate an outing to do something they like, whether that be a movie, a museum visit, dinner, a cup of coffee, whatever. If you want to go this route you should probably either talk to their primary caregiver or even directly to your friend to arrange the details.
As far as actually purchasing items, some others have given you some great ideas already. You could go for something like one of those 501 places to travel type books to help them plan what they are going to do once they are better and give them something to look forward to. Or even a book about one of their favorite topics, but in case they are having problems reading I would go more towards a coffee table type book that has lots of pictures in addition to text. The Netflix gift subscription MomToOne mentioned is a wonderful idea as well. Due to some of my side effects, I found puzzles very useful even though I wasn't really a puzzle type of person beforehand. Some art supplies, like acrylics, brushes and paper, or even coloring books and pencils, as a creative outlet.
Like I said in the beginning, just seeing you and knowing that you are thinking of them will be quite a gift in itself. Hope this information helps you a bit.