[She] knew, if he guessed the truth, that at last he'd got one up on* her.
* Get one up on somebody - WordReference Forums
I'm having problems translating what a list of old scores it must settle. The problem I have is that I know what it means, but that there aren't any good equivalents in my native language. So, would it be possible to say the same thing in a different way? Comments, please!
His look seemed to say that he knew she had taken the paper
to write to her lover, who had left her as he had warned her she would
be left. She remembered the scorn with which she had turned from him
that day, and knew, if he guessed the truth, what a list of old scores
it must settle.
How about 'There are so many old debts still to be repaid' People must have debts in your language, mustn't they?