What event in the past is she sorry about in the moustache?

In "The Moustache" by Robert Cormier, the protagonist, Mike, reflects on his grandmother's reaction to his decision to grow a moustache. She expresses sorrow over a past event, specifically the death of her husband, Mike's grandfather, which she feels is tied to the moustache. This connection highlights her emotional pain and the memories associated with her late husband, revealing the impact of the past on their present interactions.