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

I don’t think he is interested in the lady. He is interested in their diolog about the grapes. It is boring that he's staring out of the dinning-room window to the witch appear, the conversation attracts him. He has some great ideas from what they are talking.

What idea does it put in his head?  

Aunt Letty’s mentions that fruit from the land of youth make he think that he can get some fruit from some other world through the rings. Maybe the fruit cure his mother.

What does Aunt Letty say about the grapes?

Aunt Letty says,  the way grown-ups do, if there is anything to make Digory’s mother good, the fruit from the land of youth can help her. Nothing in their world will do help.