For my experience, take more interest in the condition of your passenger, than your enthusiasm for flying. Fly steadily, and avoid other than straight and level as much as possible. Chat normally with them, knowing that their chatting less (or non) is a strong indication they are not feeling well. Tell them that there's no problem going back early if they like.
Provide fresh air, and draw their attention to the horizon. If three or four times you point out something on the horizon, and they do not comment, it's time to take them back regardless of what you think about their condition.
As said, even hardened pilots can get motion sick when they are not flying. If yu want them to go again, make this time as pleasant as you can for them.