It's also possible that over the next two years, either he or his coworker could damage their relationships with investors. Typically, messy blow-ups reflect far worse on founders than politely stepping away. It often goes better to simply say something like, "Hey, I'm sorry, but I don't think we're aligned anymore. This startup probably works best with just one captain at the helm, so for that reason, I'm out."
Most importantly, even if sticking around for another year or two meant the difference between guaranteed funding and none at all, I'd still choose the extra year or two over the funding. Founders usually earn half—or even a third—of what they would make elsewhere. That extra time essentially translates to a couple of years of self-funded startup runway, and I'd much rather have that flexibility than guaranteed funding with strings attached.
Something I've learned doing a startup and talking to a lot of other startup founders is that one of the biggest risks is sticking around for too long just because there's still money in the bank. Your most precious resource is time, and if it's pivot-hell for 18 months you're well into sunk-cost land.
Bailing from a startup with significant traction or bailing when it's too early to tell could be much harder to explain away, but it doesn't sound like that's the problem here...