Today learned the severity of lowering the respect of someone in the eyes of others. If a person is looked at as an upstanding individual in his community with good character and a Torah scholar then one can’t tell the community leaders that where this individual use to live back in the day he was not of such stature. This might belittle the individual in the eyes of the community or even cause him to lose his job.
If it is a rav of a congregation even if it is true that he is not as big of a scholar then what people think he is, it is still a lack f respect to the Torah and this Rav’s own dignity to belittle him in the eyes of his congregants. It might cost him his job and the standard of Torah observance might be lowered. He is at least trying to help people observe Torah and any really difficult questions he hopefully sends to other Rabbis who might have answers.
Also many times people don’t like hearing rebuke and will decide not to like rabbis or yeshiva students who try to help them go on the proper Torah path so they will unfortunately spread rumors about them in the next place they live. This is based on a gemara in Kesubos 105b. The episode in the Gemara Bava Kama 117a where Rebbe Yochanan calls Rav Kahana a fox after Reish Lakish told Rebbe Yochanan that the lion came back from Bavel and Rav Kahana did not understand Rebbe Yochanan’s shiur as well. Calling him Rav Kahana a fox in front of Reish Lakish did not constitute lashon hara because everyone saw he was not able to answer Rebbe Yochanan’s questions and was sent back from the first row to the last row of the shiur, so by calling him a fox instead of a lion he did not lose any respect which was not lost beforehand. Rebbe Yochanan was also teaching a lesson to stay in Israel because the air of Israel makes you smarter and able to learn learn better then outside of Israel.
This is also true for a newlywed who just got married and is learning in Kollel. To say he is not as serious of a learner then he looks like will eventually get back to his in-laws and they won’t respect him as much and it will trickle down to his wife and kids eventually.
The same thing is true about any professional, to say he is not as much of an expert then people think can cause him to lose his livelihood. Even if he is a novice, he must start somewhere, and he will learn and get better on the job so if people think he can do a good job let him do it.
When you do know there can be a serious problem, for example a community is looking to hire a rabbi, or elect and official and you know if they do so it will be a big mistake, then divulging information in order to avoid potential problems will be discussed at length in Hilchos Rechilus chapter 9, b’ezras Hashem.