Reports for tribunals, if not requested by the Tribunal Service or the DWP, aren't covered under GPs' NHS contracts, so they don't have to provide them.
There are three possible options.
One is to see whether the GP will provide a report for a fee. Another is to obtain a copy of relevant medical notes and use those instead. The other option I can think of is to see whether someone the client has seen in secondary care would be more accommodating.