Few things satisfy as much as giving advice and having it taken to heart. You feel wise and the one who benefitted believes he’s been granted good counsel. Sometimes, though, we’d rather feel in command or victorious than wise; so we might offer self-serving advice and wind up deceiving rather than enlightening.
The Torah considers that to be on par with “set(ting) a stumbling block before a blind person” (Leviticus 19:14), i.e., it’s like “placing a figurative ‘stumbling-block’ before someone ‘blind’ to anything” as Ramchal explains it. That’s to say, it’s like tripping up someone who trusts you and couldn’t imagine you’d fool him like that.
What we’re expected to do is to “pass on the clear and unadulterated truth to whoever might come to you for it”, regardless of “whether you’re going to benefit by the outcome or not” Ramchal explains.
That’s difficult, to be sure, which is why “so many people stumble when it comes to this every day” Ramchal admits. Because many of us have too “strong (an) urge for profit” and prestige. But “an honest person’s duty when someone comes to him for advice” he submits “is to offer the self-same advice that he’d give himself, and for no other reason than for the good of the person asking for it.”
There’s an exception to that, though. If the intentions of the person asking you for advice are to do harm somehow with the advice you’d offer, then it’s “certainly a mitzvah to deceive him. As it’s said, ‘With the perverse you act perversely’ (Psalms 18:27)”.