I realize this goes against medical dogma, but since Nephritis causes you to excrete excess protein (proteinuria) it would seem quite reasonable to me to use a relatively easy to digest protein source (i.e. protein powder) during a time when your body is in need of amino acids to rebuild muscle tissue (peri-workout).
Further the source of the protein does not matter all that much (as long as its a complete source) when you get down to talking about amino acids. It is likely that your doctor told you to stop using protein powder because it is somewhat vilified by the medical industry; I doubt he intended you to go out and double up on steak to make up for the lost protein. Basically, he wanted you to lower your overall protein intake to help manage your condition based on the current clinical guidelines and that lowering can come from reducing your intake of powder, real food, or both.
But alas I am a bioengineer and not a doctor, so I can’t really say. (though my mother is and she and I argue the topic of protein consumption quite often; I’m making progress )