Hilchos Lashon Hara, chapter 2, section 12
If someone delivers a talk in a beis hamidrash, halacha forbids one to mock him and say that his talk was worthless and that there was nothing to hear. Through our many sins, many people transgress this and do not consider such mockery as forbidden at all. But according to halacha this is an absolute prohibition, as it causes one’s fellow financial damage and pain and embarrassment. Even if this were true, lashon hara is forbidden even about the truth. If one is truly concerned about the speaker, he should discuss the matter with him and point out his mistakes. By giving this advice he will fulfill, “You shall love your fellow as yourself.” It is certainly forbidden to make a mockery of him before people.