Patient co-insurance and balance billing are 2 different concepts. If the ONLY reason the patient had $15.07 responsibility was due to credentialing issues by your practice, and is a one time thing, not a consistent waiver of patient responsiblity, then waiving the $15.07 is likely perfectly fine. If the patient would have had a co-insurance even in network, then it should not be waived unless it meets your practice's financial hardship policy.
Co-insurance means the patient owes a percentage of the approved amount.
Balance billing means the patient is responsible for the difference between your billed charge and the insurance approved amount.
Example: CPT 12345 is billed with a charge of $200. Insurance approves $120. Policy pays 80% of the approved $120 for insurance payment of $96 with patient 20% coinsurance of $24.
Balance billing would be billing the patient not only the $24 co-insurance, but ALSO the $80 difference between charge and approved amounts.
There are a lot of restrictions on balance billing in certain situations due to the federal No Surprises Act as well as state laws.
In your situation if the patient would have owed the same $24 coinsurance or more if credentialing had been completed, I would not waive the $24. If the patient would have had $0 responsibility if credentialing had been completed, I would waive this one instance. If you have many patients in this situation, the waiver could make it seem as you are creating incorrect claims to the insurance and I would want to involve someone with compliance or legal experience in the matter.