Sefer Chofetz Chaim Chapter 4 halacha 11 part 2

If you don’t tell the person you are questioning that it’s for a shidduch, business partner, hiring a new employee, etc., for your own wellbeing then asking him question is transgressing “placing a stumbling block in front of the blind” because if he answers any questions negatively it is obviously with the intent of speaking lashon hara and he gets a sin even if he is helpful to the inquisitor.

The source for this concept that one is liable for a sin even if he didn’t actually commit it but they had intent to commit a sin is based on a Gemara in Nazir 23a which says that if a person was trying to eat pork chops and he wound up eating kosher lamb chops he still needs an atonement because his intent was to sin.

More severely, the Gemara in Bava Metzia 58b list 3 types of people that go down to Gehinom and never come back up, one of them is a person that calls another person a derogatory nickname even if the guy got used to being called that and is not embarrassed anymore, still he doesn’t come up from gehinom because his intent is to embarrass the guy. So to this person answering questions even if he winds up helping the questioner, if he is not told that the questions are being asked for one’s own good then he is obviously answering just to speak lashon hara so even if it is helpful, his intent is bad and he will be punished for speaking lashon hara therefore in order to not put him into that position you must inform him that you are just asking for helpful advice.