She would have to be able to demonstrate this was a loan and it was intended from the beginning this would be paid back.
Such examples are a contract between the two of you; evidence that you had made some effort to repay it such as a standing order to her bank account; or references to you paying it back at some future point - such as her saying, 'When you pay back the �1700 I'll put it towards a car'.
It will be very difficult for her to prove this in the circumstances you describe - if there had been no intimate relationship between you it would be easier.
Tell her you owe her nothing and see what happens.