Your girlfriend will need to file a police report in the town in which she currently resides. The possibility of prosecution will depend on a number of factors but please keep in mind the police will question her on why did not report it earlier. If she didn't tell anyone about the molestation and he still lives across the street, there will be quite a bit of questioning. She needs to be prepared to bet met by a reluctant police department. If this is the case, she needs to continue to push the issue and if she knows of other victims, to bring them up as well. She should also meet with other law enforcement agencies (like the U.S. Attorney's office) in case his molestation also covered child porn, as well.