What R Best Volumizing Hair Prods 4 VFine, VSoft

What are the best volumizing shampoos & related products for very fine, very soft, very straight hair that is available in the US.

My sister has really fine, really straight hair. She's tried a variety of volumizing products and gets some volume for a couple hours, but it tends to fall out pretty quickly.

She is willing to spend on more expensive products IF they really work. So cheap or expensive is ok as long as they really work and won't make hair frizzy.