Even with no tenable claim at all, the absent cofounder has a gun to your head: a proposed liquidation that pends on a civil suit simply isn't going to close. There are time limits on all of this stuff, and the acquirer is simply going to say "fuck it" and walk if they can't predict when the deal will close. A detail I think people who've never sold a company don't realize is that the legal costs for both sides of a deal that closes uneventfully can get close to 7 figures.
I guess this is valuable information for startup founders. It's also a reason you should run, not walk, from any early-stage business partner that wants to negotiate or complexify vesting. 1 year cliff, 4+ year vest, the way everyone does it, or go start a different company.