How do you manage sudden regression in the performance of an experienced colleague?

I’ve been in an acting team manager role for the past six months. I’m relatively new to the team compared to others, and I’m also the youngest. Recently, I noticed that a senior team member (who works reduced hours) has been missing deadlines and seems to be struggling with their workload, possibly due to personal commitments.

A stakeholder recently contacted our senior manager about a missed deliverable, and I found out that this team member hadn’t even started the task by the due date, nor had they communicated with the stakeholders. I want to address this issue without coming across as rude or confrontational, especially since many of my colleagues have been in the company longer than I have.

How can I manage this situation diplomatically while maintaining a good working relationship? Any advice would be appreciated!