I can't tell you where it should be reported because I don't know all the details of how or why your client received cash compensation. If they are self-employed it should be reported on Schedule C. Other than that you will need to read IRS instructions to determine where to report.
@Kappaalex1 - folks ask questions of varying complexities on this forum. The question that you have asked is one that should not be asked by someone that charges a fee for tax preparation. For the sake of your clients and your own professional reputation, please search out some tax educational materials before continuing to work with these types of clients. Clients go to professional tax preparers because they aren’t competent to prepare their own return. It is only fair to our clients that we are indeed competent to handle the services that we are being compensated for.
Ukraine - hang in there