That's completely separated in many places. Someone could easily step on that and it will completely give way and come off, creating a jagged edge. I don' think there's anything you can do short of removing the detaching section and building up a patch that would make this structurally sound again. Caulking is probably a bad idea at this point because that will give the illusion that it's firm and someone will be more likely to step on it. Also, with the gap already that wide I don't think caulking is going to prevent it from continuing to separate.
Because the crack isn't extending farther into the coping, I don't think you're in danger of it spreading to other areas. The way it will get worse is just to continue to widen until that whole section falls out. But, the first step in a long-term repair will be to remove the cracked part anyways. I'd say be safe about it and make sure no one steps in that area but there's really not a good temporary fix until you're ready for the longer term fix of removing the bad piece and putting in a patch.