What causes a serious change in Mrs. Dempster's condition?

Unfortunately, Dunstan makes the grave error of telling her one day that he has located Paul. In Mrs. Dempster’s mind, Paul remains a ten year old boy, and the memories come flooding back like a crashing wave. Despite Dunstan’s attempts to explain the truth to her, Mrs. Dempster constructs a delusion wherein Dunstan has kidnapped her young child. In the midst of a violent meltdown, she tries to scratch his eyes out. Both he and the doctors agree he should not visit anymore, though he stays in touch with the hospital to learn that her mental and physical heath regresses from this point onwards.