Witnesses are of the utmost importance in emergency systems since they can trigger timely location-based status alerts. However, their collaboration with the authorities can get impaired for the fear of the people of being involved with someone, some place, or even with the same cause of the emergency. Anonymous reporting solutions can encourage the witnesses, but they also pose a threat of system collapse if the authority receives many fake reports. In this paper, we propose an emergency reporting system that ensures the anonymity of honest witnesses but is able to disclose the identity and punish the malicious ones. The system is designed over an online social network that facilitates the indistinguishability of the witness among a group of users. We use a game-theoretic approach based on the co-privacy (co-utility) principles to encourage the users of the network to participate in the protocol. We also use discernible ring signatures to provide the property of conditional anonymity. In addition, the system is designed to provide rewards to a witness and his/her group members in a privacy-preserving manner.