In the past several weeks, I haven't been able to find of the Whole Soy yogurt anywhere. At Whole Foods, I was told it was a supplier issue and that they would have it back in when it was available.) I couldn't find plain Silk Soy yogurt either. Trader Joes used to sell large tubs of soy yogurt, but the've been out also. (I didn't ask there, but I suspect it's the same supplier that is rebranded for TJ).
The Whole Foods in Pimmit Hills had Silk Soy Yogurt in fruit flavors. If I recall correctly, they had strawberry, blueberry, peach, and lemon. I'm not sure if you can use a fruit flavor for her challenge.
My younger daughter and husband are also allergic to milk, so it's been very frustrating not to be able to find soy yogurt.
If you find any, can you report back? I'd appreciate it!