The GPU Temperature has nothing to do with it.
The problem is his GPU not the System.
I had this problem on Gigabyte RTX 2070 before on my i7 8700. Even when i set 100% GPU Fan Speed, it still turn black screen.
I have 2 Computer to test it on..
Gaming PC - i7 8700/32gig ram/Windows 10 1909 - All Updated including Bios + Latest Nvidia Driver . = Black Screen at 100%.
HTPC - Ryzen 5 1600/32gig ram / Windows 10 1803 - Windows Update Disabled + Latest Bios + Latest Nvidia Driver = Same result black screen.
I also tried Uninstall Nvidia and use older nvidira driver. I get same result.
I Ended up RMA it and got replacement RTX 2070. Plug-in the new and No more Black Screen.
Best way to RMA is call them and be angry!