If *I* were in your shoes.... Start the eviction process and rid the apartment of peanut butter. In the meantime, load kiddo up during the schoolday/week with peanut butter (since they have worked with you to accommodate your daughter's needs). On weekends, no peanut butter at home - do you have family/friends who would help you out by letting you keep PB at their home and come by to feed your daughter? Will your daughter understand that having PB at home could make the little boy very sick - possibly die? That might help to get her on board for now.
Just spit-balling...