Which character is becoming Miss Sullivan's ally at the end of Act III, Scene II?

At first, Captain and Mrs. Keller are uncertain about Miss Sullivan's methods in teaching Helen.  At times, Captain Keller wants to send Miss Sullivan back to Boston.  He does not believe that Miss Sullivan can truly help Helen, who is deaf and blind.

In Act III, Captain Keller begins to see Miss Sullivan and Helen making progress.  He sees that his daughter is responding to the teacher's methods.  Captain Keller expresses his gratefulness to Miss Sullivan, saying that she has "taken a wild thing, and given [them] back a child."

Captain Keller agrees to help Miss Sullivan by being consistent with Helen.  He plans to establish boundaries with his daughter.  He will no longer give in to all of Helen's whims.  In the past, Captain Keller and his wife had let Helen do almost anything she wanted.  For example, at mealtime Helen ate off of everyone's plates with her hands.