If so, what a shock that must have been to her.
- "What a/an X" is an exclamation.
- "must" refers to a necessity.
But what's the meaning of the sentence(like sentence above) that combines them together?
The fuller text is here:
Beverly has changed her seat so that she is no longer sitting beside her husband and so that she doesn’t have to see Dana’s shape beneath the sheet. So this is what has become of their weekend away, for which she’d had such hopes. Her marriage facing imminent ruin. Stuck in an isolated hotel in the thrall of a deadly ice storm, without power, sharing the lobby with the corpse of a woman who may have been pushed down the stairs by her wealthy fiancé. If so, what a shock that must have been to her.