The risk owner should implement the secondary risk response, as it is now being tolerated, and update the project documents accordingly. They should also update and communicate the assessments of the secondary risk's impact to ensure everyone is aware of the situation.

According to the PMI-RMP Handbook1, the risk owner is responsible for implementing the risk response plan and monitoring the risk and its secondary risks. Therefore, the risk owner should take the following two actions when the secondary risk emerges:

Implement the secondary risk response and update the project documents. This action is consistent with the risk response strategy of tolerance, which means accepting the risk and its consequences. The risk owner should execute the planned response for the secondary risk, such as contingency plans or fallback plans, and update the relevant project documents, such as the risk register, the risk report, and the lessons learned register, to reflect the current status and impact of the risk.

Update and communicate assessments of the secondary risk's impact. This action is consistent with the risk monitoring and control process, which involves tracking the identified risks, evaluating their impact and probability, and reporting the risk information to the appropriate stakeholders. The risk owner should reassess the secondary risk's impact on the project objectives, such as scope, schedule, cost, and quality, and communicate the results to the project manager and other relevant stakeholders, such as the sponsor, the customer, and the team members.

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PMI-RMP Handbook1

PMBOK Guide, 6th edition, Chapter 11: Project Risk Management2