...unless that's something OP could be sued for. There's no way to know without reviewing the communications and agreements (either verbal or written) between the two founders.

Which, again, is why it's insane to try to get legal advice from internet strangers.

You need money to sue. Unless the company is worth millions it would be financially irresponsible to sue over something that has no value.

OP is pursuing this because he expects the company to have value someday, which means the potential for a lawsuit is a problem.