As she saw the future in Pennywise’s deadlights in the summer of 1989, Bev learned of everyone’s adult fates, no matter how great or sad they happened to be.
As the unfortunate suicide of Andy Bean’s Stanley Uris is revealed during the earlier moments of IT Chapter Two, it’s also revealed to the group that Bev Marsh has always possessed the knowledge of her friend’s death.
While the group was upset with her for keeping this knowledge to herself, Stan’s letter in the finale shows how right she was to have kept this information to herself.