What a sweet doggie! I'm sending prayers right away!
I still don't understand how this could have happened. I know that while grooming, dogs can act differently due to the stress of being around all of those strange sounds, sights, feelings, etc...but does your vet not have a dedicated grooming room for this? The vet I worked at kept all dogs seperate at all times, especially during grooming for just this reason.
Have they notified the owner of the St. Bernard? I don't think it is their fault, but they should be notified of how stressed their dog is during groomings there and may seek a different alternative from now on.
I'm so sorry this happened. Prayers are on the way!