Just an idea...call the cops and tell them that your friends can identify some of the people involved in the crime. The cops can question the girls and threaten to put them in jail for the crime unless they 'fess up to who did it. Cops are good at intimidation!

If the cops can't (won't) do anything, then find out where they live. Wait a couple of months, then go over some night and pump "liquid nails" or super glue into their door locks, and trunk lock.