If you have a patient registration form that did not state any insurance info disclosed by the patient, that document along with the receipt for full payment made by patient stand as a proof that there was no insurance fraud committed by you.
Aetna's Timely filing limit (as you may be knowing) is 90 days. If there is still time, please confirm eligibility and try submitting it to the payer. Upon receipt of payment from Aetna, you may decide on refunding to the patient. If the TFL is already lost, the patient has no right to claim it from your office.