It's not a question of "how much effort" but more "what kind" of effort. In my situation, I'm getting a firm impression one of the cofounder is "checking out"[1

It's not a question of "how much effort" but more "what kind" of effort. In my situation, I'm getting a firm impression one of the cofounder is "checking out"[1] so the kind of effort needed to notify his fellow cofounder is probably not something he can summon at the time. I find it particularly distressing because, in my view, your cofounder is like your lifelong partner. Imagine going to another country without telling your husband or wife for a week. Maybe it'll be fine if you apologize afterwards... but what would you tell your kids? Metaphorically, the "kids" are everyone who sat in on that meeting and were amazed at the lack of communication between the two. [1] He doesn't really seem to have a huge role in the company anymore. He does not seem anywhere near as stressed as his cofounder. He kept silent when there was a "we don't have product market fit, oh shit" situation when it was revealed to the company.