A leader from Project B repeatedly asks a team member on Project A for help. What should Project A's project manager do?

The project manager should discuss the situation with Project B's leader to address potential impacts on both projects. This proactive communication helps manage resource allocation, prevent scope creep for the team member, and maintain inter-project cooperation. Ignoring the Project B leader or allowing the behavior to continue could negatively affect Project A's progress and the team member's workload. While ensuring the team member completes their committed work is important, it's a reactive measure; the root cause of the cross-project requests needs to be addressed collaboratively with the other project leader to find a sustainable solution.