The biggest thing for me is that she's eroding his emotional sovereignty. She's taking covert actions to modulate and decide his mood for him.
Sometimes, when I'm feeling down, I just want to feel that and get through on my own. But she's deciding which of his moods isn't appropriate and is changing his behaviour. If this were out in the open, he would be able to accept or refuse her attempts to cheer him up or divert him. But he (presumably) doesn't even know it's happening. That's not cool.
It sounds fine because it's worded like she's helping him but she's still taking away his autonomy. Just bring it out in the open: "hey, I've noticed, when you're sad or stressed, peanut M&Ms cheer you up. Would you like me to keep some on-hand?" With that, you've alerted them to behaviours about themself and got their consent to "help" them.
If that's the timbre of their interactions, I've got no qualms. But setting the context as "I train abused dogs" brings the mental image to one step above "hiding medicine in a dog treat."