Good shout.
https://forums.geforce.com/default/...external-display-using-dvi-d-to-hdmi-adapter/
Really need to test on another monitor with another cable if you can borrow one. Or a TV.
It's obviously sensible to test another screen, but the direction of the connection here is what makes the difference.
GPU --------> Monitor
HDMI --------> DVI-D
If it's like that then all it is doing is converting the physical connector, since HDMI is a digital signal only, and unless the monitor he has is a fake DVI port, and only allows DVI-I (odd if it does), then a new monitor is needed.