Of course it's a spoiler if she hasn't played it.
Let's step through the logic:
Darque hasn't played game,
someone tells her something about the game,
now the surprise (and possibly her enjoyment) is spoiled.
Hence: spoiler.
<{POST_SNAPBACK}>
I think the situation is compounded by the fact that she said "Hey, I'm playing NWN at the moment."
It's one thing to talk about it in an open discussion and given its age, I wouldn't lose much sleep (any sleep) if someone mentioned it without hiding it in a spoiler tag.
But when someone says "Hey I'm playing the game," it's a bit bizarre to just go out and state straight up what the end of the game is like. Even if she already has been spoiled from previous discussions.
If someone where to pick up Ultima VII ( a 13 year old game now ), I wouldn't just blab the ending suddenly to them.
<{POST_SNAPBACK}>
One thing everyone knows (even if they don't know who the heck she is) is that aeris dies