1.In Chap. 7, why is Digory so interested in the lady who brings some grapes for his mother?

because the dialogue between his Aunt and this lady make him interested

And he knew that really were other worlds and that he himself had been in one of them

2.What idea does it put in his head?

He thought he can go to other world to get fruit by ring and maybe the fruit from other world would really cure his mother

3.What does Aunt Letty say about the grapes?

What lovely grapes and it would make Mabel feel little better but if it from other world maybe can cure her