Trust me, I had the same problem.
No matter what I wore, I'd get bad sweat stains.....almost instantly. Thats why my closet was 95% black clothing, and 5% white clothing since they hide sweat the best. I dealt with this for about 7-8 years and it was horrible.
But for the past 2 years, my life has totally changed. I dont sweat a drop anymore ! The first time I went to the bar and didn't sweat was amazing ! havent looked back since.
How ? Drysol ! I dont know if you can find it on the shelves in the states, but I know here in Canada, Shoppers Drug Mart carries it in the pharmacy.
Be warned, its some pretty harsh stuff. So, the first few times you put it on...Put It On Lightly !! or your gonna have a rough night.........Put it on every second night before bed for a while....then u can use it less and less. She might have to go through a tough and painful first few nights...but its worth it. And it works instantly.
So yea, Google Drysol..get the dab-on one.
People suggesting strong deodorant have no idea. But as a helper, I use Mitchum for deodorant because it has the highest aluminum chloride content. Drysol is 40% and a liquid.