What you do can have long reaching implications for the Watchtower Society. The police need to be alerted, but I recommend that you do not over react and make the situation worse than it is.
The ideal solution for the victim is for the elders or the sister to call the police. I would recommend that you tell your girlfriend that you are legally obligated to tell the police, but would rather she did. If she or the elders do not you will have to.
I also recommend you document everything. What do the elders say and do? Are they going to act legally or will they follow Watchtower protocol. The information you find out over the next few days will teach a lot about the WTS and can possibly be used to help others know whether the Watchtower Society is learning from it past mistakes. It is by people such as yourself making a positive stand that the Watchtower Society is being made to sit up and listen and rectify their inadequate peophile policies.