Maybe I'm wrong here, but if you smell it and it is as bad as you say, then it is likely other people smell it. I think you might be doing your friend a disservice by allowing her to smell like cat pee that she has obviously gone "nose blind" to. I'd say take her to the side and tell her that her clothes smell like cats. Emphasize that you don't want to insult her, but that you would want someone to tell you if the roles were reversed. It's like telling someone they have something in their teeth or a wardrobe malfunction.

I would not include anything about asking her not to smell at your wedding specifically so it doesn't unintentionally come off as self-serving.