Is it haram to say that a non muslim or disbeliever will go to jahannam?

Assalamualaikum, is it haram to say that a non muslim or disbeliever will go to jahannam. I am pretty sure it's clearly written that a person who dies while not believing in Allah or associating partners with Allah will go to jahannam, so it's clearly writen there but I heard a story from a Mufti that, there was once 2 muslims, one was a person who would always try to do good deeds and other one used to do bad deeds, the good deed one said to the other guy to stop doing this sins, but that bad deed guy didn't stop and then the good deeds guy said that, he will go to jahannam, when they both died, Allah put the guy who did good deed in jahannam and one with bad deed in jannah, the reason why he put that good deed one in jahannam because in a way, he told that he can decide whoever goes to Jannah and jahannam and not Allah, so if I someone says something like this to a disbeliever and not a believer like in this story, will it still be a sin?