Disclaimer: I am not a lawyer.
While there doesn't sound to be any doubt about who was liable for the initial incident, I guess the issue is the delay. She could claim that as you couldn't see any damage at the scene, said you would inspect the car more thoroughly at home and didn't get back to her later the same day / the next day to notify her of any damage you must have damaged the car yourself sometime in the next few days and are now trying to blame that on her. I have no idea how that would play out with the insurers, but I'd be wondering if it's worth the hassle / delay and potential increased premium at renewal if you submit a claim rather than just fixing it out of pocket. Maybe get some costings for repair / replacement and then decide?
Note that strictly you should notify the insurers of any accident regardless of whether you are making a claim or not, whether or not you do that is of course up to you.
