Hey B. I have been using the ON Casein Chocholate and it tastes amazing. I don't even mix it with milk and it still get a thick consistency if you don't add too much water. I've also used the Cookies and Cream by Matrix, but I really don't like that sort of flavour but my sister who liked it said it was great. I've got no experience with the ISS product unfortunately.
So try and figure out what her flavour preferences are, and if she likes chocolate I would say the ON is a good bet, or if she likes the C'n'C give the matrix a shot. Also I've heard some people that are quite happy with Cytosport's EvoPro.
Good luck with it