Like I said Callisto was just ahead of me and turned onto the Bridge of Free Return but was gone when I got to the bridge and still gone when I caught up with Narathzul on the other side.
In any case it doesn't matter any more. While I was looking around the area for what happened to Callisto I accidentally hit "c" and my current spell was a "recall" to a mark some distance off. By the time I got back to where Narathzul was waiting Callisto was with him and I was able to proceed. I'd guess that he indeed did fall off a cliff or through the bridge and eventually worked his way from out of the ravine to where Narathzul was at.