Who is The Man Lewis Saw After Ten Years' Absence from A Grief Observed and what is their importance?

This man serves as an unpleasant object lesson to Lewis of how much the impression and memory of a person can change over time, and how quickly and perfectly seeing the actual person again can correct the warped memory. Lewis imagines in dismay that his memory of his wife will warp toward his own tastes and lose the unique qualities that made her her own.