I think my problem is that I was hoping the mom would get all that MM said from my semi-incredulous look.
I'd have grabbed a kitchen timer, told the kid, "Ok, your turn ends in 3 (or whatever seems reasonable) minutes. When this rings, that means it's Ellie's turn." Then at point, if he was still unwilling, I would have told him, "This does not belong to you. You have had your turn," and taken it. That, of course, leaves the screaming kid for his mother to deal with, but after an entire childhood of being bullied myself, no way in H-E-double-toothpicks would I be able to stop myself from it." Possibly it is a good thing I don't have kids. Then again, I'm all the time dealing with "Don't talk out loud!" "You can't tell me what to say!" conflicts at the group homes.