In this book, Aslan says, "did I not explain to you once before that no one is ever told what would have happened?" (emphasis his).

Aslan seems to be forgetting the time, in The Magician's Nephew, when he told Digory what would have happened if he had stolen an apple. Getting sloppy, Aslan...

Ah, but here lucy did sth. wrong, and aslan didn't want to tell her if it would have been alright if she hadn't done it, because its already to late, anyway.

In the Magicians nephew, diggory had acted right, although it was hard for him, and aslan told him what would have happened if diggory had made the wrong decision - it would have turned out bad. So aslan is strengenthing diggorys decision and moral, by saying "although it was hard on you, it was good not only for narnia, but for you personally, too."