It seems unlikely that you would be in hot water over this, or even that your department would discover that you 'knew all along'. I would ask myself about my motive -- is it more a 'compulsively ethical' streak of reporting violations? Is it driven by an already established competitive relationship with this student?
And beyond that, I would ask yourself if you would like to be notable to your department for pointing out another student's ethical violation (particularly one that does not impact you). If her violation does not impact you, it could simply get you entangled in an another student's financial and ethical issues.
People who generally behave unethically and speak openly about it tend to sink their own ships along the way.