Hello, thank you for the question.
This might be best reserved for your college or professional association, as far as your professional obligation in dealing with your client.
However, there is a legal answer too. If you told the client that you did, or had done, or would, file the return on the client's behalf, and instead filed it later after the due date so that the client incurred penalties and/or interest, then the client would be well with their rights to seek that back from you. As well, if the late filing disentitled the client to tax credits, deductions, or some other benefit and you knew about it at the time (which it likely would be assumed that you did or should have) then the client has a claim against you for that as well since the client is out of pocket due to professional negligence.
I assume that it's something like this, which leads to you asking the question here. These things tend to fester if the client is allowed to stew, leading to complaints about you, bad ratings online, like that. If this is the case and the client is making noise, then either settle it quickly with the client or you'll likely be under an obligation to report to your governing body and your professional liability insurance program if the dollars at stake are meaningful.
Does that make sense?