Yogurt instead of formula at 9 months?

I have a friend who mentioned that she had enough formula to last through about 9 months. After that she said she might just feed her baby yogurt instead of formula. I think it sounds insane but maybe I don't know anything. I'm expecting my first baby in December and I wouldn't do it but I'm curious, is this something people do? I have nieces and nephews and have never heard of such a thing..