I thought it was antiperspirant that leaves the yellowish stains, but I can't be sure. I would hate to see someone ruin a good costume by bad advice! Here's an url with lots of different advice including soaking them in vinegar and water.
Deodorant Stains On Clothing
She has it even trickier because the sequins can discolor. I would try to call up a prom or wedding dress maker and ask, maybe they would have an answer. Some resellers of prom dresses and wedding dresses would know what to do too.
Good luck and good advice to check before you add anything that might stain more.