It looks as though you've already been given links to most of the sites - you have the Tanya's site forum there and the Facebook page I'm a member of.
As far as slowing the progression is concerned, two of the key factors are adequate hydration and controlling the cat's phosphorus levels - new research seems to indicate that controlling phosphorus is more important than controlling the level of protein in the diet. With that said, the best diet for a CKD cat is the one they'll eat - all the other treatments are for nothing if a cat starves itself to death! The gold standard for keeping a cat hydrated, especially as the CKD progresses is sub-q fluids; however, I know there is a general reluctance to prescribe those in the UK. So you do what you can - add extra water to canned food...as much water as the cat will tolerate. And encourage drinking - cats will often drink more from a water fountain than they will from a bowl for instance. Some cats like to drink from a glass or a cup too. Anything that encourages water consumption is good. The aim for controlling phosphorus is to keep it fairly low - well below the maximum level on a lab value which also has to allow for the much higher phosphorus levels that kittens have. In early stage CKD, the aim should be to keep phosphorus below 1.45 mmol/L. If the cat's phosphorus level is higher than this, you will want to feed the lowest phosphorus food you can that also contains a decent amount of high quality protein - the rx diets are very low in protein and contribute to muscle wasting. What I do with Rosa is feed the lowest phosphorus diabetic-friendly diet that she will eat and supplement that with aluminium hydroxide as a phosphorus binder - given with food, it binds to the phosphorus in the food and prevents the cat from absorbing it. The liquid that vets often prescribe is mint flavored and cats HATE it. You can buy the unflavored powdered form online. Something else to have tested is whether or not the cat has proteinuria - not all do, but if a cat does then benazepril (Fortekor) is often prescribed to help bring that down. And regular blood pressure checks are very important as CKD cats tend to have raised blood pressure which, of course, can cause damage if it goes untreated. If your friend hasn't already had a full blood chemistry done, then that would definitely be worth doing. A lot of CKD cats have low potassium levels which can cause their own set of issues as well as the obvious risk to their general health. Rosa's IM specialist says that he likes to keep potassium all the way at the high end of normal range where possible because he's seen numerous cats lose a lot of potassium very quickly so he likes to put them at a level where they can lose some without crashing.
Most of the other treatments are, as Jayla says, based on decreasing symptoms. CKD cats are prone to excess stomach acid which can often be controlled with either famotidine (Pepcid AC) or ranitidine (Zantac). If your friend notices that her cat is vomiting white foam, that is a symptom of excess acid and would give a good indication that it might be time to start one of these treatments. It is important to control excess acid because if it isn't controlled it can lead to bleeding in the GI tract which of course can cause anemia, which again CKD cats are prone to as the CKD progresses as normal kidneys produce erythropoeitin which stimulates the production of red blood cells. As the kidneys become less effective, they are less able to produce enough erythropoeitin so anemia can become a problem for them. If you do suspect a GI bleed, then Sucralfate is a good option for allowing the GI tract to heal. Though I will say that if that becomes necessary, ask for the pills not Carafate liquid - Carafate is artificially flavored with something that cats seem to find unpleasant (I believe it's meant to be a bubblegum flavoring, but that is only from what I've found on the internet - I didn't taste Rosa's, I promise)!!
Cerenia or Ondansetron can be used for nausea if that becomes an issue for kitty. And Cyproheptadine, Mirtazapine or Zyrtec can be used as an appetite stimulant. A lot of vets will give a stimulant without an anti-nausea med, though I am not a fan of that idea - I can't even imagine being made to feel hungry while feeling nauseous, so it isn't something I'd want to do to a cat either!
B-complex and B-12 are both good for improving how a cat feels overall and B-12 can also help to slow down the onset of anemia so is definitely worth considering. They are safe to give as they are water-soluble so any amount that kitty doesn't need will just be peed out.
As you know, we can't 'heal' CKD, but we can control it. A fairly big part of how fast it progresses is down to genetics and the particular course the illness takes for an individual cat, but we can help to slow that progression by adding the right treatments at the right time.
And one more thing. It's very easy to drop into a pattern of anticipatory grief knowing that this is something we can't fix. Our cats live in the moment - all they know is how they feel right now. Please tell your friend to try to remember that...and to keep doing all the things that her cat loves. For Rosa, that's going outside for walks and exploring and then coming back in the house and cuddling and purring. She doesn't know she's ill and neither does your friend's cat - and that's the way we want to keep them feeling for as long as possible. Cats still want to be cats - restricting the things they love too much for fear of what might happen is as damaging to their quality of life as the illness is. I would love to wrap Rosa in bubble wrap so nothing can hurt her, but she would hate that...she wants to explore and run around and chase bugs, so that's what we do - carefully and supervised and on a leash, but we restrict what she does as little as possible.
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Then again, this was the same vet that took months to diagnose Rosa's bladder stones because she's female and was the wrong age to have them...he's probably lucky I don't live there any more because know I know her CKD is at least partly caused by a kidney stone that no doubt showed up around the same time, I'd have to be restrained from doing him some real damage!!