It just seems to me that Charlotte is taking some secret pleasure in Lizzie's being slighted by Mr. Darcy. She rebounds quickly and makes the comment about wishing he had danced with Lizzie, but I think it's to save face for herself more than it is to heal any wound Darcy may have inflicted upon Lizzie.

In Chapter 6. Charlotte says the following: (the underlined parts are my own emphasis)

"It may perhaps be pleasant," replied Charlotte, "to be able to impose on the public in such a case; but it is sometimes a disadvantage to be so very guarded. If a woman conceals her affection with the same skill from the object of it, she may lose the opportunity of fixing him; and it will then be but poor consolation to believe the world equally in the dark. There is so much of gratitude or vanity in almost every attachment, that it is not safe to leave any to itself. We can all begin freely -- a slight preference is natural enough; but there are very few of us who have heart enough to be really in love without encouragement. In nine cases out of ten a woman had better show more affection than she feels. Bingley likes your sister undoubtedly; but he may never do more than like her, if she does not help him on."