I have no such problems with Siege, but suffered it a few weeks ago with Dishonored 2 on PC. It was a, say, 4th such happening in mine 20+ years gaming history, so it is usually rare, but happens.
General explanation is complex and deeply subjective, connected with the general condition of eyes and sight, as well as game design and engine. Also the position during gaming, distance from the screen, and many other factors can contribute. My other such situation was with the (latest) Thief - I have this game on PC and PS3 too, on PC sickness starts sometimes, only during the long gaming sessions, say 1 hour or more. On PS3 it happens always, after 10-15 minutes. I'm PC gaming with the monitor 50cm from the head, and PS3 gaming with the TV that is ~4m away.
Your friend can try to change his usual gaming position, FOV change can also be tried, but I'm afraid that at the end he will have to find another game to play...