You are being a good friend and that is what counts. One of the problems I find is that the british dont tend to talk openly about things that matter, and then when you are really guessing about what you can do you can get things wrong without meaning to. I have cancer for the second time now. When I had ovarian cancer nearly 20 years ago, I found a mixture of attitudes. Many friends were keen to be helpful, but sometimes it did the opposite. So I am a singer and was a keen gardener. So if I met a friend at a garden or in the choir , or hadnt seen them for some time, then would ask how I was and how the treatment was going on etc. This was caring of course, but if I was engrossed in the music or the plants, I was actually having that little "holiday" from being conscious of the cancer. I tend to face things and thought about this and then I made what seemed a bit of a shock to friends but did work out well. I simply said "I dont have cancer at the weekends" which they used to then look surprised and wondered how to respond. But I would then go on to explain that I appreciated how much they wanted to show they cared etc but that if we were having lunch or gardening or whatever it was good to just enjoy that time together, and not have the cancer overtake everything. Some found it a bit odd but then it really worked. They knew what I wanted , and didnt have to wonder if they should ask how I was or what. It actually made it easier and more relaxed for everyone. They still asked how I was of course, and it allowed me to tell them if I was having a bad time.
So , if you have known them for some time, I would suggest you think about anything that they particularly enjoy. So for example, perhaps they enjoy swimming. You might find out which swimming pool has good disabled access and if they have a chair to put someone into the water (I use this myself) . This might mean your friend can still swim , when it might be a worry trying to go down steps etc. So you are looking to see what areas and interests are worth looking at , to try and estimate what if any help might be needed or wanted. Obviously things will change as time goes by, and they may need more help. It is a balance, as you also dont want to look too far in the future and start talking about when they cant drive or whatever.
I drove hospital car for over 10 years and heard from many patients things that they wished their family had offered to do for them, but they often wouldnt ask themselves. So again, it is not trying to crowd someone or offer to do to much, which can backfire as from what you say they begin to realize , if they havent already thought about, the problems that are likely to arise. At the moment, I would suggest that the most important thing you can do is to carry on keeping touch as you always have done.
You know the person, so perhaps at the moment it is partly the shock and a depression looking forward thinking of all the negative things and almost mourning what you will not be able to do in the future. They may also start to be self conscious and worry about holding other people up because they are slow walking, or worrying about sitting in public in a cafe in case they might spill something etc. It is a balance of trying to help them to carry on as much as they feel able to continue to do themselves, offering the sort of help that is needed rather than vague offers. So, for example you might say " X garden is open next sunday. Would you like to come with me, and I can pick you up.?I have been there before and the garden is lovely and plenty of seating. So that way you have offered an outing, made it clear that she should be able to get round as much of the garden she feels up to visiting etc, which while tactically covering the problems she has, does not mention the actual illness all the time.
I do hope these ideas give you some ideas, but what is the absolute rock bottom is that you are still her friend. She is your friend with a health problem, not a health problem who used to be your friend. Well done you for thinking of her and what you can do, and you are doing it already