I can't understand why that would happen. I use yogurt all the time, and have always had great results, ie, light and fluffy cakes. How many recipes has she actually tried with yogurt? Perhaps it's the recipe(s), not the yogurt?
An alternative to yogurt is buttermilk - these two ingredients are basically interchangeable in baked goods, as is sour cream. Maybe she can try the buttermilk and see if she has better results. If you don't have access to buttermilk, you can substitute sour milk, ie, 1 Tbsp vinegar or lemon juice added to a 1 cup measuring cup, then filled with enough milk to make 1 cup sour milk. Let stand about 15 minutes, then stir.
"Strength is the capacity to break a chocolate bar into four pieces with your bare hands - and then eat just one of the pieces."