This is a fictional story concerning what happened to Susan. It's a little better than what Philip Pullman seems to have assumed. It looks as if J. K. Rowling has the same problem as Pullman with respect to Susan.
See also this from letter to children: HT: Jason Pratt
From another letter to a boy named Martin: "The books don't tell us what happened to Susan. She is left alive in this world at the end, having then turned into a rather silly, *conceited* young woman. [emphasis mine] But there is plenty of time for her to mend, and perhaps she will get to Aslan's country in the end--in her own way." (ibid, p 67.)