Honestly, I have no ideas. The site admins have said you can email them to appeal an unfair hellban, but normally that's for the hellban-ee. It would be awesome if they added a feature to upvote people back from hell. Frankly, in this case the user doesn't have any contact info or about me, and he's not that active (and he has negative karma).
My big complaint is that this pattern of habitually telling everyone they're hell-banned seems to contravene the will of the mods, and produces a bunch of low-level noise on the site. It's come up before, and the mods continue to use hell-banning as a punishment so obviously they think it's worth the risk of losing casual commenters.
If you see a case where you very strongly disagree I think it's appropriate to comment, but this doesn't seem to meet that standard.
This guy hasn't said anything that deserves a hell-ban.