Launcher shows a white window

Hey Guys, my friend is getting issuses when he tries to start minecraft. The Minecraft-Launcher won´t show up correctly: only a window with nothing in it.
LOG:

[0131/171703:INFO:LauncherAppBrowser.cpp(24)] Work dir is C:\Users\Eigene Dateien\AppData\Roaming.minecraft

[0131/171703:INFO:ClientHandler.cpp(182)] Creating launcher window.

[0131/171703:ERROR:child_process_launcher.cc(533)] Failed to launch child process
[0131/171908:INFO:ClientHandler.cpp(149)] Shutting down!

We have also tried to reinstall Minecraft or Java, opening the Minecraft-Launcher with someother Modlaunchers (like Curse) also won´t work. Its in modded and unmodded Minecraft (because it´s the launcher), Java 8. So he´s very sad that he can´t play it anymore with the "normal" launcher