I had it bug on me my most recent playthrough in exactly this way. I didn't have a workaround. I just saved prior to doing it (I do a lot of saving because I've been having more CTD's since Trespasser came out) so I was able to reload and try again. This time instead of range killing him (I was a mage) I got inside his camp and killed him up close and personal and Cass said her line, the quest updated and I got credit. Maybe you need to be right in his face when you kill him? I made sure after that to do it every time this way (he was the first) thinking the whole thing may require you be in the camp when killing the target.
Success.
I left the Exalted Plains and came back and went to visit Ser Hildy's camp ... and he respawned. This time I kiled Hildy up close and personal ... and I got credit this time and the quest marker disappeared.
... Another question from the same area.
I cannot get the snow white wyvern to spawn. However, I earlier gave Vivienne the ordinary wyvern heart. I wonder if this is preventing Snow White from spawning. In my previous playthrough I did not give Vivienne the wrong heart, and was able to get Snow White to spawn after wandering a round a bit.
Has any one else had this problem of giving Vivienne the wrong heart and not getting Snow White to spawn?
Am I doomed to have this quest marker and incomplete comanion quest taunt me til' the end of my days?