Quoting lil*Mama*Bearô:" Everyone we are eating when MIL is around she always gives my son food. Not because I didn't already ... [snip!] ... this too where her in laws do things that they don't want but no one can speak up. Is that normal? I tell my parents what's up."
It is sooo hard but it is just something that needs to be done as straightforwardly as possible. He should just say it respectfully, mom, please don't feed him that, or, mom, we don't smack his hands.
She probably will getbpissed but if he just says that and sticks to it and doesn't elaborate, she will not be able to vent to anyone or retelling the story without sounding like an idiot and hopefully that will help to drive the message home.