My friend gave me the following scenario. A Fighter has the Suggestion spell cast on them with the following phrase: "You look tired. Why don't you take a seat over there." If the rest of the fighter's party is suddenly ambushed immediately after, I argued that the fighter would be able to get up and help, since the suggestion is no longer reasonable. My friend said that whether the suggestion is reasonable or not only matters when the spell is first cast.
Which of us is right? Does the Suggestion spell provide 8 hours of mind-control, provided the suggestion is reasonable at first? Or does it have to continue to be reasonable?