caution (one) about (someone or something)
To warn one about someone or something. My mom always cautions me about the dangers of texting while driving. You need to caution him about Allie before he falls prey to her usual tricks.
caution someone about someone or something
and caution someone against someone or somethingto warn someone against someone or something. Haven't I cautioned you about that before? Hasn't someone cautioned you about Daniel?