It is forbidden to speak rechilus even if it is totally true and even if it’s not in front of the person you said you heard it from. All the more so if he is standing there, and you are brazen enough to say what he said about him, even if he already knows that the guy said it, it is still forbidden and considered rechilus. Saying it in front of the person who you heard it from is worse for two reasons: 1. You are causing more hatred and setting up a worse fight if you have the audacity to tell the person spoken about what the other guy said about him in his face, makes it more believable. 2. There are other prohibitions that are very easily applied in this circumstance. There is a gemara in Shabbos 118b which quotes a personal testament of Rebbe Yossi that in his life he never said something behind anyone’s back. This could mean that you can speak rechilus behind someone’s back but the Chofetz Chaim goes into much detail to prove that wrong through gemaras, the Rambam, Sma”g, and Tosfos, many we have seen already. There is a famous Yerushalmi in Peah 1:5 which says 3 people are killed when lashon hara is spoken: the speaker, the one accepting lashon hara and the one talked about. The Rambam quotes this gemara and the Beis Yosef, Rav Yosef Cairo’s commentary on the Rambam makes it sound like it only applies to lashon hara nor rechilus, but the Chofetz Chaim clarifies that the Rambam is arguing on the Raavad , for the Raavad holds the gemara only applies to rechilus like the example given in the gemara but the Rambam in fact holds it applies to both lashon hara and rechilus.