I had a very similar situation. I had a family that joined my Bear Den this year, coming from another Pack. Shortly after joining, the mother in this family began sending out scathing emails accusing various Pack leaders of everything from incompetence to taking "pay" from the Pack funds. In person (Pack, Den meetings) she would either avoid you or act quite cheery when conversed with. Her son acted fair enough, but always acted a little odd, kind of like a person would act if he was hearing bad things about you. After 6 months of this and the inability to get this woman to come into reality I finally had my fill and told the CM the family could no longer be in my Den. She must have felt the "vibe" because before action could be taken the family moved to another Pack. I wish them luck.
As for your situation I recommend what I will do should this type of thing occur in my Den again, and that is taking the quick action of telling this family to ship out. I'm sure she is bringing your whole unit down, and destroying the joy your Den Leader has for the program. Yes, it's sad for the boy, but it'll be more sad when you lose a great Den Leader and the spirit of the unit.