It'd probably be safe and sensible for them to:
1. Remove staff from the moderation list, and outsource moderation to a neutral third party.
2. Reverse the person's ban. Reach out to them privately and directly with a personal apology. Offer one year free subscription if they wish to return to Roll20, and express understanding that they may not.
3. Publicly apologise to the community for screwing up. Say they are installing neutral external moderation on the subreddit and say they have reached out to the person banned, reversed the ban, and offered them free time if they want to return.