Since there are too many coins to mix he has 2 options.
1) Keep the 120,000, un-mixable, and currently utterly traceable coins. Then try somehow to get them out. Knowing that your every move is recorded and tracked.
2) Give them back 'most' and get to keep 'a lot' but totally legally.
If he got to keep $20 mil legal, and gave back $50 mil.. I'd do it.
i doubt that your option 2 is a viable option at all, because they will surely press charges against him because he hacked their website and stole a lot of money.
what they usually do with stolen coins is that they leave them be for a long time and then start mixing little by little.