How does Darcy solve the family crisis Lydia has caused?

There are two motives for Darcy to help the Bennets. One is that he is in love with Lizzy and wants to relieve her distress at Julia's elopement with Wickham. Another is that he believes he is partly to blame for the elopement because he had not exposed Wickham's nature, which he had failed to do out of personal pride.