I think it's far more helpful to both answer the question and remind the poster that the answer is in the standard, and that they should become very familiar with it and go to it first whenever they have a question. In the future, they'll get the answer a lot faster by looking at the standard first.
Maybe they read it and missed it? Maybe they misunderstood the wording? I try to keep an open mind. There's a way to nudge people to be better while being helpful and kind.
And of course, if your answer rubs them the wrong way (even if you're in the right), they're a lot less likely to learn from your advice, are less likely to use the forum in the future, and will feel a need to defend themselves in the thread. I think trying not to unnecessarily upset people is a virtue in its own right.