This paper aims to study how to achieve high transmission security in the physical (PHY) layer of a multi-tier heterogeneous network (HetNet) through a simple device-to- device (D2D) enabling scheme. For the HetNet, we propose a simple D2D-enabling scheme with low complexity for users to opportunistically enable their D2D mode and become either D2D or cellular users by exploiting the diversity of all user association signals from all base stations (BSs). To evaluate whether the proposed D2D enabling scheme improves the PHY-layer security of the HetNet, the secrecy outage probability of the HetNet is defined and analyzed from two different perspectives of BSs and users. We define the BS- centric and user-centric security outage events and derive the explicit lower bound on their probability when the proposed D2D enabling scheme is adopted. Our analytical and numerical results not only show that the proposed D2D-enabling scheme can achieve high PHY-layer security but also reveal how densely the BSs should be deployed in the HetNet in order to achieve the high PHY-layer security from the perspectives of BSs and users.