So he is supposed to chuck all his hard work and walk away? No way. He should roll up his sleeves and put up a fight. No one has ever succeeded by being a wimp, except windows 3.0.
It's just business.
This is why partners should incorporate and come up with a mechanism for handling acrimonious departure before getting to work. A classic response to this problem is the shotgun clause.
The early histories of many successful SV companies are peppered with a sour business deal or two. The winners are those that hung on and fought. Did Gates walk away when when Jobs discovered that he was building GUIs? Who walked away from Facebook? Zuckerberg or the other guys? You know the twins...or "those other people" whoever they were. You get the point. Exactly, its just businesses.
This "company" has no actual assets of any sort (users, contracts, relationships, press, brand recognition, ...), except for the code, which his co-founder does have a legitimate claim to. And due to the founder dynamics, the company will never amount to anything.
So these guys are basically fighting over 10% of nothing. Yup, walking away seems like the best course of action.