ZvT diamond level, find the error

  (Edited: )

Hi all,a good friend of mine (diamond zerg) struggles a lot in the ZvT lately (mid - late game). I always try to find the mistakes in his play and give him advises, but now I can not find any big mistakes any more. He uploaded his last four ZvTs on GGTracker and by just looking at the graphs he should have won in my opinion (in his obviously). In the game itself he did nothing wrong, so why did he lose?Here are the links to the games:

http://ggtracker.com/matches/4283742

http://ggtracker.com/matches/4282730

http://ggtracker.com/matches/4281382

http://ggtracker.com/matches/4278223

http://ggtracker.com/matches/4230510

http://ggtracker.com/matches/4230173

You can download them by pressing the big green "DOWNLOAD" button;)Please make your suggestions on the one hand as specific as possible but at the other hand nothing obvious like "he should have seen and killed the drop" or "he should switch races";)

Archiatrus