Had a friend who was in a flex 5v5 with both premades and he won. His whole team was reported by the enemies and was permanently banned. I took a look at his ban report, and he had said nothing.
First and foremost, reports do not stack. Whether he was reported by one player or nine, the end result would have been the same.
Secondly, did he say nothing as in not a single line of chat or did he say things that you personally don't believe were banworthy? In any case; it's pointless to try and discuss a punishment without seeing the evidence or game in question. If your friend really believes that he is innocent, have him post here or get the chat logs.
Remove the automatic ban system and hire people to overview bans please.
An automated system is the only way to deal with a game of League's caliber. Hiring a full-time team of employees that work around the clock would bankrupt the company and result in less consistent, less timely punishments.