See, this is interesting. I am going through a very similar situation. Masters degree, I lag behind assignments and I often avoid checking email for the fear of confrontation.
But, just as you self-diagnose as having depressions/burnout, I diagnose myself as being chronically lazy, therefore I take all the blame and responsibility with no exceptions. So, funnily enough, our shared predicament has cast us on opposing sides of this whole discussion. You're sure he's depressed and shouldn't be reprimanded, I suspect he's grown lazy and comes up with excuses as I often do. The more you know...
It has been hinted that communication between Marty and cbob is intermittent at best, so I wouldn't be so sure about that. Also, don't underestimate the combined power of three imaginations instead of just two, a third perspective could do wonders.
Yeah, another writer means different ideas, that's the point. And we're not talking about some Hollywood reboot where they usually kick out everyone responsible for previous incarnations. Cbob could (and should) remain fully in control, take the other writer's work, trim it down, expand upon it, or rewrite it all, using but one of this writer's ideas.