Thank you for the prompt reply. Should the CPU as well as his graphics card be fast enough ? GTX 970 / AMD Phenom(tm) II X6 1090T Processor, 6 cores, RAM: 16381MB be enough?
As said if he runs it locally it just works.

We will try it with a lower game speed.

When a specific player connects to the game he is completly unable to move and is unable to open his Inventory.
Furtermore when joining without pausing the game his catching up bar is moving backwards.
Damage is visible on his screen after around 20 - 30 Seconds. He used to be able to play just fine ...