Nobody's picked up on this yet, but it seems to me that the child's "addiction" to peanut butter needs some work.
Craving peanut butter may be signs of an underlying emotional problem that needs to be addressed.
Craving Peanut Butter: What Does It Mean? (healthline.com)
Cutting it out completely could result in withdrawal symptoms similar to kicking other addictions.
I think Martsin did note that her child was in counselling. Might be helpful in the long run. But doesn't solve the current issue of the roommate's child.