Here is an awesome site on kidney disease in cats. I can't recommend this site enough- very thorough site! Check it out!
http://www.felinecrf.com/
I'm sorry to hear that your cat's not feeling well. You've got some other options, too! You can ask the vet about doing IV fluids at home to help flush the kidneys. The vet can show you how to do it. I know this can be done, because I had a friend that gave her cat IVs for kidney problems too.
As for the Stinging Nettle, is that safe for cats??? I'm all about holistic meds, but I am too chicken to try it on me and my cats. Why don't you try posting a question about Stinging Nettle on the Ask A Vet forum. There's this very sweet vet on there named Aleda on there, she's a holistic vet and knows all about herbs. To find that forum, click the Forum icon on top of this page. Scroll all the way down until you get to pets, and you'll see the expert forum.
Please, come back with an update!!! Good luck and best wishes to you and your kitty.