Corn gluten vs. corn meal
Doug says that he can't say whether your mother-in-law is confused - he doesn't want to get into that family fight. :-)
However, he can say that corn gluten is normally used for weed control. Corn meal has been recommended for general garden improvement in some organic circles as well as fungus management.
So yes, your m-i-l is doing something positive for her garden but she's not stopping weeds. Does that make her "confused"? Not me - boy - I'm not getting into that conversation. :-)