When playing scrabble, if someone leaves an "in" to making a word that will give you a triple word score, do you take it?

Or are you polite to leave it and let them use it? This happened to me and of course I took it, and the person I was playing with got very mad and said they had "built out to there" to use it and I shouldn't have. I thought the idea of playing a competitive game was to win.
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