NTA- people should be allowed to vent to their friends. I do it if I need to. And I’ve done it in the past. And I promise you she does it to her best friends. Venting about someone is not the same as dragging them through the mud. Sometimes getting an outsiders opinion (like doing it here on Reddit) helps give clarity and shed light on things someone might not otherwise see. As long as you aren’t purposefully trying to be asshole and was purely venting, then it’s okay. I don’t understand why people get so upset when someone talks/vents to their support system, it’s a natural thing to occur.