My opinion on the subject.

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What you have presented here is not necessarily a fallacy, because you have only presented a conclusion to an argument.

Fallacies generally occur when there is an error in how the premises interact with each or an error in how the premises are used to come to a conclusion. There is an argument that any statement that comes from pure emotion is fallacious within itself, but that is pretty hard to prove in real life.

The conclusion may be weak, but as you have presented, we can't really evaluate how fallacious the argument is without any premises.

As to the validity of his claim, your response that the players could have a strategic imperative not to tell the DM all plans is a good one.

Or you could start researching hard line non-consequential ethical theories such as Kant's deontology and explain that while hardline rules are theoretically effective, they do not work well in an imperfect world, and would lead to situations in which people may be unnecessarily hurt (I kid, I kid).

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Summary: No, I don't think the conclusion within itself is inherently fallacious, but I cannot determine this without his premises. It is likely a weaker argument, however.