Upwork is a red herring in this case. As frustrating as it may be that Upwork wasn't more helpful in resolving this case, the current state of affairs is as follows:

1. The author (freelancer) did $12.5k amount of work that he wasn't compensated for

2. The owner of the CC that was "accidentally" charged $12.5k received all their money back through bank dispute

3. Upwork was able to claim their cut for the $12.5k worth of transaction that occurred between the author and Robin

4. Robin received $12.5k amount of work without paying a dime

Robin needs to pay the author back the $12.5k he is owed. If Upwork isn't willing to help reclaim the debt, he needs to sue Robin directly for the amount. It's very strange how much good will the author is willing to show towards Robin given how Robin is the only entity in the story who got away with all the spoils without paying a thing.