The only reason I think Jill might have befriended Susan before Susan stopped believing really is the quote I posted. It just seems weird to me that a girl who only had an outsider's view of Susan would be
so hard on her. It actually would be quite rude to deride Susan in front of her family unless Jill had also once been close with Susan and then Susan fell away from the group.
Eustace is family and thus can complain all he wants. Polly is an elder who can, with the wisdom of her years, exclaim over "kids these days". Jill, if she never really knew Susan well and wasn't a friend, does not really have a right to complain about her. (and I can't imagine Susan post-belief associating closely with one of her cousin's friends whose only connection is Narnia)
Of course, it all could be that Jack needed
someone to say that paragraph, and Jill was the one through process of elimination: Eustace and Polly already had their paragraphs, and Polly spoke for the point of view of her and Digory's generation. Peter doesn't seem to want to talk about Susan ("My sister Susan," answered Peter
shortly and gravely, "is no longer a friend of Narnia." emphasis mine). Lucy can't speak because in the next chapter her first words are specifically noted. I
could see Edmund saying Jill's lines, but I think he would be like Peter and not really want to think about Susan not being with them.
I am possibly putting way too much thought into this.
The Lord is my light and my salvation, whom shall I fear? The Lord is the stronghold of my life, of whom shall I be afraid? - Psalm 27:1
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