• Is changing your HWID against SOE ToS and can you be banned for doing so?

Yes. If you get caught, they will perma ban you. They also dont just ban by HWID. They use multiple methods simultaneously

• Is there no 2nd chances (yes, he's an idiot for even trying to cheat but a failed attempt doesn't really make you a cheater but an idiot)?

That depends on the ban that was issued. Some are single account, some are perma multi ones.

• Can a player be banned if it looks like the player knows too much? We always talk on Mumble while playing and I can imagine that many has clicked "Report last kill" thinking we was cheating, not knowing we actually had communications between us.

No. They only get banned depending on the contents of the server logs and any evidence seen by the CS staff.

• Can he demand and expect some sort of evidence which can prove his guilt or at least some info explaining better than just "violations: Cheating in H1Z1"?

Not a chance. It would defeat the entire point of having an anti cheat system if they let you know what triggered the ban.

In the CS's eyes, and almost every player here, 'Once a cheater, always a cheater' is the mantra.

The best way for him to deal with the ban, is either accept it and move to another game, or appeal to the CS department, and ask what type of ban is in place and if he is allowed to purchase and use another account. As i said, some actions result in only a single account ban. Others, depending on severity result in a perma ban for any account you purchase. he may get away with avoiding detection right now if he thinks he can try and change his HWID, but once the automated system is in, he will find it much harder. Especially if they use the same systems as in PS2. The tech there bans the account before your game even loads up.