Good luck with your decision, ideally she herself should come forward and advise they are partnered. But you need to do what is right for you, she chose to make it known they she is illegally receiving payments which has put you in an awkward position.

She needs to be aware that if she is found out then she will be prosecuted, and even though she was with her partner, as she is the one receiving the payments she will be the one that has to pay back all of the debt, and it can quite quickly add up to many many thousands of dollars.