It's not okay to betray someone that trusts you.
Tracy is dating Felix when Felix gives her one of his credit cards and its PIN in case she ever needs emergency cash.
Tracy wants to buy some new clothes.
Tracy uses her own credit card to buy herself some new outfits.
Tracy uses Felix's credit card to buy herself a thousand dollars worth of new outfits.
Felix uses his own credit card to buy Tracy a necklace for their six-month anniversary.
Felix dumps Tracy when he discovers she's used his credit card for clothes and makes her pay him back with interest.