First, cutting split ends will reduce the frizziness more than any product will be able to. If you haven't already, suggest a trim.
As for frizz-fighting products, the majority of the ones I know of involve a lot of silicones which can make hair look gunky in excess. Her choice of shampoo and conditioner may also need to be changed. If your hair is super-oily/flat, you'll probably want to avoid stuff labeled "intensive moisture" or anything like that. Look for something volumizing.
However, I have very oily hair and I manage just fine with most hair products that do use silicones; I think the key is applying as far away from your roots as possible and using as small an amount of product as you can get away with. Also, apply on wet hair and then blow dry (if the hair is straight. if it's curly, let it air-dry or blow dry with a diffusor).
L'Oreal's Smooth Intense serum, Citre Shine serum, Biosilk's Shine Spray and Beyond the Zone's Frizz Zapper are all great, affordable choices that smell good too.
