Girl threatening false case after breakup—how to respond?

I have a question: if a girl is threatening to file a legal case against a boy — both were in a casual relationship and had consensual physical relations twice, with no commitment for the future — and the boy had clearly informed her (with written proof via chats) that he wouldn’t marry her, which she had agreed to and even fixed her marriage with someone else — they haven’t been in contact for a year, but now suddenly she is threatening him and asking for money to settle the matter. What should be done in such a situation?