Even though there is compelling evidence that the lashon hara heard is definitely true you still can’t decide that it is for sure true without doing you own investigation even if the person being spoken about is right there in the room and stayed quiet. That is no indication whether what is said is true or not. Maybe the person decided it’s better to stay silent then to exacerbate a fight or he thought there is no point speaking up because no one will believe him anyway.
A proof to the ffact that silence is not an admission when it comes to lashon hara is from a halacha in Shulchan Aruch (Even HaEzer 178:9) that if one witness testifies in court that a woman was adulterous and the husband believes him as two witnesses then he should be concerned and divorce because he has a right to make himself forbidden to her. But we don’t absolutely believe this one witness to cause her to lose her kesuba even though a woman proven to be adulterous does lose her kesuba. Also, if he doesn’t trust what the one witness said he doesn’t have to divorce his wife, even if his wife was silent when the claims were laid out against her. Even according to the Maharshal brought down in the Beis Shmuel there who says her silence is an admission that’s only in this case where such a claim deserves an answer if not true since it has such severe repercussions but in general an insult or claim does not have to be answered, in fact the gemara in Chullin 89a says that the world is worth being in existence because of those who are quiet when an argument ensues.
So if a single person who testifies in court of such a serious matter as adultery, which if he is lying he is putting his whole integrity on the line, destroying the life of this woman, and having her husband divorce, and she was quiet but still the Shulchan Aruch says we can’t decisively believe what he said, all the more so anyone else who speaks lashon hara even if he has the Chutzpah to say it in front of the victim that still does not give the listeners a right to believe it. Why should you believe a bad person who does the sin if lashon hara over the assumed kosher status of the one spoken about?! Even if he had a right to speak to warn others of a possible threat still those listening must only be concerned and investigate.
Furthermore, even if the victim being spoken about is a person who always speaks up and defends himself against any attack, like for example someone like President Trump, but this time he is quiet still that is not an indication that what was said was true because there can be any number of reasons of why he was quiet as stated earlier. The Rema (Even HaEzer 2:4) says that if A person was called a mamzer/bastard to his face and was quiet that is not an admission and even if he was called a mamzer/bastard and he defended himself then right afterwards was called a challal/illegitimate kohen and was silent that still isn’t an admission. It makes no difference whether it was claimed in a heated argument or not, and if it wasn’t during an argument then the best people can do is just be suspicious and do research if need like for shidduch purposes, etc.
The Chofetz Chaim concludes in his footnote that all this is obvious but he feels he spell it out so that people won’t people tricked by their yetzer hara/evil inclination to believe any slander no matter how compelling the initial evidence is, without an investigation.