The uBitx V6, with Nextion display and KD8CEC software, does have a screen saver mode. This is accessed via the Control Menu, LCD button. The LCD Setup screen allows setting display brightness, and also provides the option to have the screen Always On, or blanked after 3, 10 or 20 minutes. It seems like that should satisfy your friend's screen blanking requirement.
I think the KD8CEC software utilizes some computing power inside the Nextion display for some additional functions (not only display related). Therefore, you might unexpectedly lose some important functions by disconnecting the +5V power to the display.
