I find it difficult to believe that someone "can't cook". I tend to think the real reason they "can't cook" is simply because they don't want to or are not interested in it. Whatever the reason, it is best to be up front about it in a constructive manner if it is something you would like for her to do.
There are tons of simple options in the frozen food section where all you need to do is heat some meat and pour on the bag of what-not and let it simmer. That is at least one step up from boxed meals.
Also, if you don't have one, I highly recommend getting a crock pot. For sure she should be able to cut veggies, pour in broth or your favorite beer, add a nice chunk of beef (or even chicken or turkey breasts) and let it cook itself!
Most of all, it sounds like she does a shitload of work around the house. Not everyone is perfect at everything. If she doesn't get the hang of the cooking thing, at least you'll have a clean house