How to prevent unjustified bans?

First of all, this isn't a ban appeal, I won't use names at all here.

A friend of mine got banned for something abit unjustified:
He was leading a VoA25, and one guy joined the raid as a tank, without stating any MS change on the rolls.
Later on, my friend found 2 other tanks to join, and asked the first guy if he can change to DPS, obviously he could still roll as MS tank as he originally joined.
The guy changed to DPS, still not claiming to change his MS roll.
They downed Toravon and he dropped dps pants for the class of that guy. As you can guess, the guy rolled on those pants, and had the highest roll, but as he was written as MS tank on the RL list, my friend gave the pants to the second highest roller, while telling it in chat.
My friend is no *****, he took screenshots of everything during the raid, as he always do in pugs he lead, yet he got banned for 60 days without even asking him what happened.

Is there a way to prevent such bans?
Ban appeals take so long that most people just prefer to wait the ban out.
If for example he did report the incident himself with screenshots the moment it happened, would have that prevented the ban? Or do the GMs read each ticket separately?

He could've kicked the dude out if he refused to confirm his MS spec.
It could've prevented the ban depending on whether the GM would read his ticket first. They read each ticket separately but they read them in clusters.
All he can do is open a ban appeal with counterproof and wait unfortunately.

Reporting first would make no difference more than likely, even more so if there was no confirmation and agreement before both sides regarding the received loot changing. There's no "obviously" there beyond a player in a DD role rolling for a DD item. It's the raid leader's duty to make it clear what sort of loot the player who changed role was eligible for, as well as to give a reminder about it when handing out the loot that could be suspicious to avoid reports in bad faith.

If someone doesn't change his loot MS, isn't it obvious he is rolling as the role he joined the raid as?
I mean by that logic, you can kinda say the RL was put into a trap.
If he let the guy take the pants, he could have gotten reported by the 2nd roller, with proof that he never changed his MS.
If tank gear dropped and he didn't let the guy roll as MS tank, he could have been reported by the guy for same reason.

And just to make it clear, if someone wanna be an *******, it's no hard to just show a screenshot of the rolls where he wins and doesn't get the item without showing any prior conversation. So even a confirmation that he is rolling as MS tank wouldn't have changed my question, How do you prevent being banned for unjustified stuff? Do you have to report after every raid with screenshots? Is there a punishment for people who report stuff if it later found out to be lies or you just assuming that people won't do that?

Again, it's the raid leader's duty to make sure there's no ambiguity. If there's nothing explicitly stated that the loot was or wasn't changed, it defaults to people rolling for their current role - that's the only obvious thing. About your last paragraph, that's where you make a reminder about the change before and after giving any dubious item like that to avoid bad faith. You keep calling it unjustified, but all I really see is a raid leader that slacked on their duty. Your question about consequences for fake reports should be directed to a GM.

New information:
According to my friend, the guy literally asked to roll as tank when he joined.