You missed another possible cause, is she being molested by someone in the household?

It's nice to read a column with such good, level-headed advice.

Re: "Retired"s comment, I don't get that vibe at all (of being molested). It would be really unusual for a child who's being molested to sit outside her mother's door wailing and screaming, where other family members (including the presumed molester) could hear; when children are molested they tend to want to hide and be unseen, i.e., to fade into the walls. Something may have happened to her long ago, but I doubt that anything like that is going on in the present. More likely, she's processing something from long ago, some separation fears or something similar like the mother suspects. Children tend to act like she's acting, when they feel safe enough on a basic level to process some ancient fears. I would agree with the columnist's advice to let time pass and let the situation resolve itself.