Well... it's hard.
What I had to contend with was slightly different -- AXH was actively trying to put a wedge between me and the kids by saying bad things about me when the kids were in his care. My approach was to focus on telling them the truth when they were with me. I never cheated on your father. I will never be angry with you if you come to me with things he has said and ask me if they're true. Etc.
For me, it was another one of those things I could not control. Like I couldn't control if he made sure they had lunches when they were staying with him. Or like I couldn't control if he made sure they went to bed at a decent time.
My therapist kept telling me that the only thing I had control over was MY relationship with the kids, and how WE interacted. And that creating a healthy home and showing them I love them is the best way to provide them with a model of what a good relationship looks like.
She said "You can't make him behave towards them like you'd like him to. You spent 20 years trying to make him behave like you wanted him to. How'd that work out for you?"
I think in a situation like yours, if you show in action that you don't get mad if she keeps the fridge open -- you just calmly explain to her that the food goes bad if the door is kept open for long periods of time, and you'd like her to remember to get what she needs and then close the door -- she will learn that whatever Dad says, Mom will not get angry at her for minor things.