He is a resident of reality and can obtain knowledge through the use of his perceptual-cognitive apparatus, that is, his brain and his body. The very essence of reasoning is the process of drawing non-contradictory inferences from consistent premises. If one's underlying premises are self-contradictory, all bets are off. Further, formal logic is not equivalent to reason. Formal logic proceeds without meaning and without intentionality.
Human minds reasoning are at no such risk.
In other words, reason allows people to derive new true claims from preceding true claims. Nothing else is asserted. By the way, there is no dark side of reason, not even in one's emotional life. Consider all of the times another person did something that upset you. If your anger was rational, the action that upset you was not.