Here's what happens:
* Elizabeth enters Victor's room, finding him in a state of terror and despair.
* Victor, unable to cope with the horror he has created, throws himself into Elizabeth's arms and cries out in anguish.
* He doesn't speak to Elizabeth about the creature specifically, but his emotional state and the presence of the creature in the room strongly suggest something terrible has occurred.
Later, Victor does confide in Elizabeth about his creation, but this happens after the creature has already murdered their brother William.
So, while Frankenstein doesn't speak directly to Elizabeth about the creature in the scene you're likely referring to, his actions and emotional state communicate the severity of the situation.