You have an outage in your Compute Engine managed instance group: all instances keep restarting after 5 seconds. You have a health check configured, but autoscaling is disabled. Your colleague, who is a Linux expert, offered to look into the issue. You need to make sure that he can access the VMs. What should you do?

The correct solution is to disable the health check and add your colleague's SSH key. The health check is causing the instances to restart continuously because they are failing the health check. Disabling it will allow the instances to remain running, even if unhealthy, giving your colleague time to connect and diagnose the problem. Adding his SSH key to the project-wide SSH Keys ensures he has secure access to the VMs. Granting the project Viewer role is insufficient as it only provides read access and does not allow SSH access. Performing a rolling restart will not resolve the underlying issue and the instances will continue to restart. Disabling autoscaling is irrelevant here since the question states autoscaling is already disabled.