The next worst thing to losing your own horse is waking up to a call from a dear friend to find out that they have lost theirs. I was lost for words, because from experience I know there is literally nothing you can say to make it any better. Losing your partner and friend destroys you, end of story. So I said what I had found most useful when I lost Cam; that it was perfectly okay to cry as much as needed for as long as needed, because they were a family member and worth a good grieving. The only other thing I could manage to come up with was that I was sorry, and that it sucked, both of which are sincerely true. This is about all I suppose you can say in this situation, but it feels woefully inadaquate.