I was thinking the same thing. Google saids Dog can go without water for about 3 days same time as humans. It was always my understanding that 3 days might be able to be extended under ideal conditions but not by much. 7 Days really doesn't seem likely.

I have to think the dog ate snow or had access to a small source of water. Maybe being on a cliff you would have a small water run off as the snow melted during the day?