If someone is paying a JD, and the JD has given a written opinion backed by E&O, then the JD can eat any and all costs if the item needs to be filed later. For your client, give your opinion as well, documenting the client refused to file the form.
I get something similar maybe once a week now (used to be multiple a day) that >2% S Corp shareholder/employees are fine reporting their company paid medical insurance only on a W2, when the las says it is wages (which means it has to be reported on constructive receipt, which is some amount every single paycheck. Similar with the owner/employees who pay themselves randomly, if at all, and not meeting what anyone would consider reasonable wages.