The Mother Ghost was restless in the Land of the Dead, floating around the surface before passing by the Gatekeeper of the Dead. She was unable to move farther while her son, Ali was lost in the Land of the Living. She gave Alexander her ghostly handkerchief to give to Ali so that he might be able to find his way to her. Though Alexander did give the handkerchief to her child, he was unable to learn what became of either ghost.
It can be assumed that Ali reached the Land of the Dead and that the two crossed the River Styx.
The longer end path of King's Quest VI: Heir Today Gone Tomorrow takes Alexander past the Mother Ghost, he must take her task on.