First we can get some terminology out of the way so we are on equal footing 
There are 3 types of penalty and one type of automated catch all.
- Ban - means they can’t play the game at all - permanently. Applied by a GM for repeated or egregious activities.
- Suspension - means they can’t play at all for a limited duration. Applied by a GM.
- Silence - means they can’t access some social features. Applied by a GM after checking logs attached to the report.
- Squelch -is an automated social chat limitation in response to lots of reports designed to deal with spammers and gold sellers. It escalates it in queue for GM review at which point they decide if a penalty (those above) needs to be applied.
Which is what the GM tried to tell you here
If they got banned it took a LOT for a GM to apply that penalty. The GM was correct in pointing that out. I am thinking that maybe the person got a Silence due to his chat lines being reported as spam or for breaking the rules (inappropriate topics/language).
No, you were politely told that they can’t discuss someone else’s account with you. You would not want them talking about your account with someone else either.
The player who was actioned with a penalty is the one who needs to appeal. https://us.battle.net/support/en/article/2639 Tell your friend to do that if they don’t agree with any penalties applied to their account. It gets the GMs to check the logs again and verify the penalty was warranted.
If people are being annoying in chat they need to report any chat violations via right click then put them on Perm Ignore.
If a player or group is engaging in persistent harassment then the impacted player needs report for that. Good overview of that here Spotlight on: Harassment
Keep in mind, people who are active in chat channels tend to draw attention. If they engage in spammy “games”, chat patterns, or content that most of the other players don’t like, the GMs may go with the majority who don’t want to see that. A Silence doubles in length each time it is applied so the person may want to review their chat activity and decide if they need to tone it down or adjust in some way.