The Gemara in Eruchin 15b makes a very ambiguous statement. Rabba says any statement that is said to the guy’s face is not lashon hara and Rabba proves this from Rebbe Yossi who said “I never said anything and then had to look behind my back.”
The Chofetz Chaim said that many people took this Gemara out of context to mean that if you are willing to say something to the guy’s face whether good or bad it is permissible. He proves this assumption wrong based on many Rishonim including the Rambam, Tosfos, Smag and the Rabbeinu Yona.
First off, besides lashon hara if one is willing to embarrass a guy to his face like being up a story of a baal teshuva in his early years or even discuss what someone’s relatives use to do before they changed there way for the good, that is also the prohibition of onaas devarim, making a person feel bad.
Even if you saw a person do a sin between man and Hashem like breaking Shabbos you can’t just hate him and publicize his sin to the world even if he is right there in front of you. Rather you should take him aside and gently rebuke him.
Even if he was rebuked properly and he did it again where there is now room in Halacha to hate him it is still not an easy matter to spread what he did to others even if he is around as we saw in the case in Pesachim 113b of Tuvia sinning and Zigud who saw this going to Rav Pappa’s court to testify what happened and Rav Pappa gave Ziggud lashes for coming in as a single witness which is lashon hara. Testimony needs at least a pair. The Chofetz Chaim does mention that one is allowed to speak lashon hara of a group of unruly people to warn others to stay away from them but that is not what the Gemara in Eruchin is talking about.
Therefore as of now we have not come up with the exact case in Eruchin but we will find out next week when we finish the footnote, b’ezras Hashem.