Even if Jeremy had signed a stock agreement, he wouldn’t have reached the standard 1-year cliff for founders to vest any equity
Sam, are you recommending that co-founders with no salary be subject to a 1-year cliff? I have always argued that it's a bad idea, and today's case is the perfect example: if someone who is not paid leaves before 1 year, they receive absolutely nothing for their work. That's not right. I'm fine with a 1-year cliff for early employees who get a salary (see FAQ section of http://foundrs.com)
To play devil's advocate, you need to hear both sides of that story. Of course one month is not much time. But if I'm the one who said the one magic sentence that made Elon Musk fall in love with Mars and told him how to get there, and I can truly claim that without me, SpaceX wouldn't exist, do I deserve something? Hard to tell. By default, if two people start working on something, they are partners, 50/50. That wouldn't be right either, but maybe the truth is not a 100/0 split.
EDIT: after having read the legal complaint, it sounds like that person was not an original co-founder, but someone who had some discussions after the startup was incorporated. The only "oops" moment is having listed him as a co-founder on YC's application. You can imagine the scene in the courtroom: "did you or did you not list Mr X as a co-founder in the company's YC application? If so, are you lying to us now when you say he is not a co-founder or were you lying then?"