What should you do if your guinea pigs have dandruff on their skin?

This probably isn't dandruff, it's most likely to be mites, if your guinea pigs have scabs on their skin, i reckon it could be. Take it to the vets for a professional opinion and they will give you some treatment for it.

Did you wash her with flea shampoo? This can cause dry skin and dandruff with piggies.

could be scurf, trademark sign of a few conditions, it is best to take your guinea to the vet if there is a problem they will give you a special shampoo or some ivermectin to treat it and it should clear up in a few weeks