What happened to the girl he let go ?

Yes, in the book, the Ghost of Christmas Past shows Scrooge a vision of Belle many years later at Christmas time, happily married to another man, and surrounded by loving children and grandchildren. A scene omitted from most versions, because as pointed out in a note to an annotated edition of A CHRISTMAS CAROL, as the Ghost was only supposed to show Scrooge visions from his actual past, he could never have seen this scene. Some versions have this shown to Scrooge by the Ghost of Christmas Present instead. In the Alastair Sim version, she apparently never marries, and instead becomes a nurse (which back then was no means as respectable as it is now), and does much work making poor peoples' lives happier.

Yes, in the book, the Ghost of Christmas Past shows Scrooge a vision of Belle many years later at Christmas time, happily married to another man, and surrounded by loving children and grandchildren. A scene omitted from most versions, because as pointed out in a note to an annotated edition of A CHRISTMAS CAROL, as the Ghost was only supposed to show Scrooge visions from his actual past, he could never have seen this scene. Some versions have this shown to Scrooge by the Ghost of Christmas Present instead. In the Alastair Sim version, she apparently never marries, and instead becomes a nurse (which back then was no means as respectable as it is now), and does much work making poor peoples' lives happier.