The reason you didn't find information about further recourse in this situation is because there is none within the ACH system. If a corporate return is submitted after the deadline the RDFI may ask the ODFI to accept a late return. The ODFI has the option of accepting it or rejecting it. However, in your case, it sounds like the ODFI is not open to this. So, at this point, the only thing your customer can do is to deal directly with the originator to try to get their money back.
--A bad day at sea is better than a good day at work.