I found some dry shampoo in my bathroom and I thought my mom bought it when she went to the grocery store last week. However, after I used it on my sister's cat I noticed it expired 12 years ago and I don't know what to do. I just wanted him to stop smelling like a dead fish.
Expired or not, do not use dry shampoo on your cat. Depending on how long ago you applied the shampoo, try wiping it off with a damp cloth to get as much product off as you can. Your cat may be fine, but please watch her closely the next few days and take her to the vet at any sign of discomfort or illness.
Most cats (breed or ability exceptions) bathe themselves and don’t require a water bath like dogs do. They bathe by licking their fur and skin. Dry shampoo isn’t cleaning the cat, it's instead covering it in product that it will then be licking up when it does bathe. It is not meant for consumption, it’s meant as an oil absorber for your hair. By putting the stuff on your cat, you may be masking the scent, but not cleaning the cat and instead potentially causing harm when it inevitably tries to clean the shampoo off its fur.
If your cat smells bad, try to instead identify why the cat smells and prevent her from getting smelly.