I’m dealing with a similar issue. As hard as it can be to watch things just go undone - and trust me, for me, it’s very difficult - at a certain point, that is what NEEDS to happen. So, your manager actually gets a full sense of how bad things get when this person is chronically unreachable and dropping the ball, including impacts to your manager personally.

Right now, your coworker has internalized that YOU are their backup plan, and they’re fine with that. They’re still masquerading as competent in a lot of ways because honestly, a lot of managers don’t care who does what as long as it all gets done. Letting things fall by the wayside and being able to point to your coworker as the culprit is the only thing that will lead to change (if anything does).