Hello Benno1980s,
Thank you for using Apple Support Communities. We want to help get your partner’s device up and running again. If the screen isn’t responding, try following these steps here: If the screen isn't working on your iPhone or iPad - Apple Support
1. Restart your iPhone or iPad.
2. Make sure that your screen is clean and free of any debris or water.
3. Disconnect any Lightning or USB-C accessories. If your screen works after removing an accessory, try using a different outlet, cable, or charger.
4. Remove any cases or screen protectors.
5.) If you’re unable to successfully restart your iPhone, try a force restart.
After your device restarts, you can disable Invert Colors by following this path: Tap Accessibility > Display & Text Size. Turn off Classic Invert.
We hope these step are helpful. Thank you for using Apple Support Communities. Cheers.