Matty has to live as an international fugative instead of presumed dead
It's ambiguous whether she ever actually gets caught, but the yearbook is slam dunk evidence that Ned can share with Oscar that a different woman's body was in the boathouse.
For the magnitude of her crime, she would be on a permanent wanted listed. She might have a nice decade or two of leisure but once the high tech Information age of the 2000s rolls around she'd eventually be apprehended.