The ROI of investing another year or two of frustration and failure so a handful of investors might have a marginally better opinion of you is poor to say the least.
There's no guarantee they'll invest in his next startup, even if he stays until the bitter end.
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.