There is no cause of action for Breach of Promise of Marriage. It is barred by Civil Code Section 43.5. If she tries to sue you file a demurrer to the complaint and you will get it knocked out immediately. A good attorney will get her complaint kicked out of court.
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I agree with my colleague that you can't be sued successfully for not marrying someone, even though that causes emotional distress.
However, you can be sued successfully for money lent and not repaid, and proof of an oral contract can be just as admissible, and the oral contract just as enforceable, as a written contract. Your and her testimony, written notes, emails, texts, witnesses to statements about whether these purchases were repayable loans, etc., are all evidence that tends to prove the claim. If your contention was that her money and the purchases were gifts to you, you need the same type of evdience, including gift cards, witnesses, emails showing intent, etc.
You can start compiling your evidence, and then hire a lawyer to defend you.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.