They suspected that he was taking the Vitamin D himself, as though he had Munchausen Syndrome. Hence the note about "the dramatic course of care."

Poisoning with VD3 is objectively weird. It's not very effective. If she had used tetrahydrozoline eyedrops, he'd be a goner after one dose -- and, with his heart and electrolyte balance troubles, odds are that nobody would be the wiser. We therefore have to surmise that she didn't want to kill him, but wanted to keep him sick, reliant, and compliant.