Sun burned ear

My sister has a cat that is 8 years old, she is black, brown and white. She has one all white ear that is turning red and crispy and curling at the end. It seems to get worse the more she spends time in the sun. Whenever she washes it or we touch it, it obviously causes her pain. Would like to know if there is any treatment we can do to help her

First, get her checked by a vet right away.  It is not uncommon for cats with pink skin to get skin cancer from sunburn on their ears and or noses.  She may already have this problem, and if nothing else, she has serious painful sunburn on the white ear.  After the vet check, if all is still well, you can consider a couple of options.  One is to keep her inside during the sunny part of the day.  If that isn't possible, another option is to use a long-acting and preferably waterproof suncreen (level 50) on that ear to protect it as best you can.  She could be sensitive to the ingredients, however, so keeping her out of the sun is the best option.