At my client's hearing, the judge said that as the applicant's Appointed Representative, I can't submit the SSA-3380 - Third Party Function Report. Is this correct?

At the hearing level, many Administrative Law Judges (ALJs) will invoke ‘Rule 3.7: Lawyer/Advocate as Witness’.  Under the advocate-witness rule, you cannot serve as both advocate (via the SSA-1696) and witness.  The ‘witness’ is required to testify on the basis of personal knowledge, while an advocate is expected to explain and comment on evidence given by others. This dual role can give rise to a conflict of interest.

In your case, the ALJ viewed the 3^rd Party Function Report you completed as ‘providing witness testimony’ which can prejudice the ALJ’s decision. Often, the ALJ will recognize that the SOAR case worker is not familiar with this rule, and educate them on their options.  When there is other evidence in the file and the SOAR case worker does not need to testify themselves, they will remain the official 1696 representative. By doing so, they are able to cross examine expert witnesses and directly examine the applicant (i.e. ask them questions in front of the judge during the hearing).