Blizzard never gives this information. While it may help your friend realize what they did wrong, it’s also prime data for hackers and botters to circumvent the system. So they just do not give specifics as a blanket policy.
I will tack on, while it’s completely my own thoughts/experiences and not Blizzard Official, a lot of the time what we see come through here on CS is people playing how they think they should and not necessarily with the whole of the group. Your friend may be guarding the tower but it’s a team effort and they need to be rolling with the team as a whole. If it’s a bum rush to the boss, they need to be with the group. Most of the times these sanctions are the result of multiple reports and instances, not just one or two BGs.
Your word isn’t the data that Blizzard uses to determine these sanctions. The look into every instance and thoroughly investigate things. That your friend already had one sanction works against them with this now being their second offense. It’s not that it doubles like chat offenses, but they’ve got a record now of this being a thing.
Ultimately, you cannot do anything for your friend. They have to appeal and appeal again until they get the official “no more appeals will be accepted” response. There isn’t any other recourse and it’s only something your friend can do. You have no place in the discussion between them and Blizzard.
[EDIT]
Adding some links from another similar case to back up what I said.
