Perhaps eliminate the video card as the root cause by trying another monitor. If not, see if you can connect using DVI (or VGA) rather than HDMI and see if that fixes the issue.
Hook it up to anything else to see if it's the panel (console / dvd player / Roku / etc). My guess would be GPU though. Is he using a dedicated card or integrated graphics?My brother has an issue with his monitor, see pictures.
Any suggestions?