To have a valid contract you need consideration: you can't get 50% for doing nothing, you have to put up something in exchange (money, skill, resources, etc).
edit: (I'm not saying anything as to whether Jeremy did so).
edit: (I'm not saying anything as to whether Jeremy did so).
Edit: reading further I guess they are arguing that none of his IP is in what GM is buying. So I guess that will be the real point of contention.
In any event, breach of contract isn't the only claim being made.
So while 1 month probably shouldn't be worth 50% of the company, you also can't just decide that you don't want your co-founder to have a stake in your company any more, and push them out.
At the end of that 1 month, they had both done equal work (about a month's worth) and had equal share (50% each). It sounds like (according to Jeremy's story) he still owns that because no one agreed to a formal change in ownership.