So, I am the SME in one area, and a backup SME in an area that overlaps. I cover my coworker's (Let's call him John) area while he's on PTO, and he does mine. A few weeks ago, he came by to let me know that he would be leaving the company as a heads up, but asked me not to discuss it with anyone. I kept silent, assuming he would give a customary two weeks notice, at which point we could start planning a shift of responsibilities.
His final day is a little over a week away, and we're getting to the point where I really should start talking to my manager to let him know my attention is going to be divided. While the two areas overlap, there is generally enough work to do in both that it will affect my performance trying to cover both areas, I would like to Inform my boss in advance so he can plan.
The thing Is, I want to respect my coworkers wishes (he didn't need to tell me). I've asked him about it and he said "He'll let them know" but isn't telling me when. How can I approach this situation?
Edit: As per Alroc's comment, dual coverage is a company standard to improve bus factor. Every person has a dedicated backup (most of the time it's a paired backup in two related areas, but sometimes not). Additionally, to my knowledge. I'm the only person he has told. I know explicitly that he has not told our team lead.