No, you’re probably not the first person to consider an alternative course of action.
But if she answers “that’s what he likes to eat” and if he, when acting on his own, also chooses to eat large amounts of unhealthy food, people might not insist on blaming her to the extent of criminal negligence. Unless he’s mentally ill, he’s got the same legal right to eat a hamburger as any of us. If he’s mentally ill, she might be able to try having him committed or getting some kind of power of attorney. It might not be the highest priority of our medical and legal systems, though.
In the case of a child, I can see possibly bringing charges of child abuse. Next time you are aware of a case like this in your area, why don’t you report it to child services and see how it goes? But people have a legal obligation to care for children that’s not quite the same as their obligation to care for even physically disabled adults.
She probably could be charged with negligence or abuse, for example, if she cut off his communication with the outside world or took complete control of joint finances. If he uses that freedom to obtain food, that’s his free will there.