In device-to-device (D2D) communications, device terminal relaying makes it possible for devices in a network to function as transmission relays for each other to enhance the spectral efficiency. In this paper we consider a cooperative D2D communication scheme which allows two nearby devices to communicate with each other in the licensed cellular bandwidth by assigning D2D transmitters as half-duplex (HD) relay to assists cellular downlink transmissions. In particular, we focus on secure information transmission for the cellular users when the idle D2D users are the potential eavesdroppers. We aim to design secure beamforming to maximize the D2D users data rate while guaranteeing the secrecy rate requirements of the cellular users and the minimum required amounts of power transferred to the idle D2D users. To solve this non-convex problem, a semi-definite programming relaxation (SDR) approach is adopted to obtain the optimal solution. Furthermore, we propose suboptimal secure beamforming scheme with low computational complexity. Simulation results demonstrate that our proposed scheme achieves a significant transmission rate of the D2D users while provides a high secrecy rate for the cellular users and facilitates efficient power transfer for the idle D2D users.