Maybe she just realizes that Darcy's interference was indeed in the service of his friend... Darcy truly believed that Jane was not interested in his friend and he believed that Bingley would be hurt in this lopsided relationship. Isn't it admirable that Darcy's intention was to help his friend? Imagine Jane was not Elizabeth's sister, but a stranger. As a disinterested observer, would Elizabeth have thought ill of Darcy's interference? I think Jane Austen does a fantastic job of describing Jane's behavior and demeanor so that we can see Jane's detachment. Correct me if I am wrong, but at the Meryton Assembly, Darcy encouraged Bingley's interest in Jane. Didn't he think her the handsomest girl at the assembly, and urged Bingley to dance with her? It was not until the Netherfield Ball, when he began to think Bingley's interest was more intense than Jane's.

I tend to think that Lizzy simply came to see the situation from Darcy's perspective, based on his letter, Charlotte's early conversations with Lizzy and her own general knowledge of Jane's personality.. In this light, Darcy not only did nothing wrong, but he probably did what most good friends would do... look out for his friend...It just so happened that in this case, it was Lizzy's sister who got hurt.

From this perspective, Darcy's error was understandable (and quite human) and forgivable. It may be that Lizzy, after understanding Darcy's intentions, forgave his human error.