I would be careful reading too much into the initial conversation. Companies have to plan for the future, just like employees do. I'd offer a different perspective to see if maybe he's over-thinking the situation. Perhaps he really did make some mistakes and should be held accountable. That doesn't mean the company/manager is trying to get him to retire.

Your FIL could be reading the situation correctly, but I've seen this type of situation so many times. He may be over-analyzing the situation based on a conversation he felt uncomfortable about, which is a fair and common conversation for an employer to have with an employee. So perhaps a change of perspective could help him see the situation and himself more clearly.