Ban repeals and the player ban system

How exactly does the ban system for players work? My friend who always jokes with people and rarely trolls around got a perma-ban, when the last time she did end up getting banned was maybe in 2013 for about 1 day when an angry player told people to report her because she and her adc lost to a Yasuo support and a Jinx. This may seem silly to ask, but it appeared out of nowhere and shook us up. Comments and such would be greatly appreciated as input on these things is usually very helpful.