In The Chronicles of Narnia what happens to Susan?

If this question refers to the fact that Susan never returns to Narnia, even in The Last Battle, it is because she stops believing in Narnia, and the truth of Aslan. It is quite clearly stated that she has allowed the things of her world to become more important to her than Narnia. Indeed, Jill (Pole) says, "She's interested in nothing nowadays except nylons and lipstick and invitations. She always was a jolly sight too keen on being grown-up." Eustace claims, "...whenever you've tried to get her to come and talk about Narnia or do anything about Narnia, she says, '...Fancy your still thinkng about all those funny games we used to play when we were children.'"

Peter states it clearly and succinctly:"My sister Susan is no longer a friend of Narnia."