Are there any phrases, idioms, proverbs in english to indicate a situation where a litigant loses his case in court because of his solicitor's professional negligent in conducting the case in court. The one comes to my mind is hard cases make bad law. But I am not sure whether it is suitable in the present context.
I don't think there are courtroom idioms that are so specific, but you might consider 'laughed out of court', which means the case presented was so poor, it was laughable.
If you want to focus purely on the unpreparedness of the solicitor, you might consider some more general idioms such as "if you fail to prepare, prepare to fail" (or other similar derivations of this).