Were they playing anything or just walking around in the demo? I have one and have tested it out with many people. When people get sick it's usually because they've been in one of the less interactive demos just sort of looking around wiggling the mouse etc. The people who I let loose straight onto Half-Life 2 or other games where there is some sort of goal rarely get the same type of nausea. I think this is worth taking into account when people say "it's nausea inducing".

For insta-nausea put someone in the Tuscany demo and move them around while they wear the rift. Hilarity ensues!