My guildie came back to the game after 1 year away and found the game close to unplayable. Before he left he uninstalled the game, but did not realise that when uninstalling it, there would still be files left.

To find these files, you need to select "show hidden folders". The ddo user files will be located under C:\Users\[username]\AppData\Local. Here you will find a folder called Turbine and another called Dungeons and Dragons Online. These filed can safely be deleted (if you're unsure, do take a backup before deleting), since they will be recreated when you start the game.

This solved my guildie's game freeze problem.