The project manager should first document the client's responsibility for the integration with the security subsystem as part of the project scope statement. This will help to define the boundaries of the project and clarify the roles and responsibilities of the project team and the client. It will also help to avoid scope creep and manage expectations. Planning the implementation, recording the issue, and informing the client are actions that can be done later, after the scope is clearly defined and agreed upon. References:
PMP Exam Content Outline, Domain II: Process, Task 1: Execute project with the urgency required to deliver business value.
A Guide to the Project Management Body of Knowledge (PMBOK Guide), Sixth Edition, Chapter 5: Project Scope Management, Section 5.3: Define Scope, Page 154.