I think it might be this one:
But that would be a tortured reading of the decision.
In brief, Judicial Watch wanted access to some tapes a historian made while interviewing Clinton, which recorded some conversations Clinton had in his official capacity. They asked the court to order the National Archives to declare the tapes presidential records and recover them. The judge ruled against Judicial Watch because a) the Archive didn’t have the statutory authority to determine what are presidential records or not, and b) the court can only compel an agency to do something they are legally required to do.
If this is indeed the case, the advice he probably gave was: you don’t have to give them anything because Clinton didn’t have to turn over his tapes! However, the classified documents came from other agencies, so even if trump determined they were not presidential records, they are still records of those specific agencies and must be returned.
Pro tip: only take legal advice from actual lawyers, not unlicensed political agitators.