The milk protein stays the same in almost all types of liquid dairy products. 
You either have to go to cheese -- predominantly casein-based, although there 
are one or two whey-based cheeses like ricotta -- or specialty commercial 
additives like whey protein to not have the complete set of milk proteins in a 
dairy food.

There's no reason to think that yogurt would react any differently on her 
immune system or her digestive system. If the doctor wants milk for the 
challenge, then it's probably the best way to proceed.

Steve Carper
Steve Carper's Lactose Intolerance Clearinghouse
http://ourworld.compuserve.com/homepages/stevecarper/

Planet Lactose Blog
planetlactose.blogspot.com