If that Brother doesn't stop this, he is not going to live long.
Cardboard boxes, particularly food packaging and shipping containers, may contain chemicals like PFAS ("forever chemicals") for grease resistance, mineral oils from inks, adhesives, and, rarely, fire retardants or pesticides. While plain brown corrugated cardboard is generally safe for composting, glossy, heavily colored, or waxy cardboard should be avoided due to potential heavy metals and plastic coatings.