Wish I had downvote powers to downvote this because it's horrible advice.
You literally have nothing to lose by just waiting for the equity to vest and keeping your 40%. Fuck the co-founder. Fuck 3%.
Worst case you end up with 40% of nothing. Walking away you end up with 0% of nothing. If you don't stand up for your equity no one will. And if you let him get away with this, down the line he'll do the same shit to someone else. Stop this guy now in his tracks, don't let him swindle you and everyone else.
Your theory of the case here being that this company has managed to find a set of contracts that establishes 4/1 vesting and enabled a seed funder to invest $100k, but somehow didn't designate any officers of the company or any authority to terminate members of the company.
It could happen! They might reasonably spend $400-$500 figuring that out.
The funny thing about this is that it isn't even good advice in the chest-puffing status-seeking model in which its proposed, because a big controversy with a former founder might mostly just makes you someone reasonable people might not want to work with.
If there's a pattern it's one thing. But for a single event when it's hard for an outsider to figure out who was in the right and who was in the wrong, it's probably easier just to avoid both of them.
I'm 100% avoiding on principle ever working with a founder who ruthlessly terminated a partner in month 11 of a cliff without simply accelerating the cliff, so there's not much need to litigate that point.
You literally have nothing to lose by just waiting for the equity to vest and keeping your 40%. Fuck the co-founder. Fuck 3%.
Worst case you end up with 40% of nothing. Walking away you end up with 0% of nothing. If you don't stand up for your equity no one will. And if you let him get away with this, down the line he'll do the same shit to someone else. Stop this guy now in his tracks, don't let him swindle you and everyone else.
Don't be an idiot.