Very sorry for your friend's loss.
To answer your question however I think we need more information.
Was the mare injured to the point of needing to be PTS by the stallion or facilities?
Was it due to poor managment while she was there (poor feeding practices ect..)?
Freak accident?
I was going to make that point too, eml....there's a far greater risk to stallions from being double barrelled by mares.
I had to drive a mare to stud over here, she was taken there, covered, and taken home again within the hour - 3 times. She didn't half lay into the stallion the first time. The stallion's owner told the mare's owner she 'wasn't ready'! Poor stallion didn't know what hit him, literally.
I don't think this accident WARRANTS any negativity towards the stud or stallion (but you know what the horse world is like!) He's an animal, not a machine... accidents happen. Sadly. Such a shame for the mare.