Firstly, examine the DMARC record on https://mxtoolbox.com/DMARC.aspx and look for any errors. If no errors seen, then there is no issue with DMARC DNS record.

There might be misconfiguration and here are the steps to check the same :-

1. Check whether the DKIM is passing on the sender domain.
2. Check if the SPF is passing on the envelope domain and also envelope sub-domain is on the sending domain.

If you find any of the mentioned issues, ask the client to fix the DKIM or SPF as per your observations.

If the issue still persist please connect with DE.