Question

Why does john want the ghost of hallock to disappear before his wife appears on the scene

Answer 1

Because he is afraid that his wife would misunderstand the situtation and feel that john was cheating her.

Answer 2

John wants Helen (ghost) to disappear before his wife Lavinia appears. She is a women who doubts him about being unfaithful to his spouse. She is a suspicious kind of lady. She is very sensitive and could not stand a sight of a mouse in front of her, so imagine what would the sight of a ghost would do to her.