I agree WildAcre. It's a sad situation all around.
Pre-existing damage aside, she had to have been snubbed down pretty darn tight and fought like heck to open her tongue like that. In a horse that was already trained and going for a tune-up, that's not the type of behavior one would expect. I'd say she was in pain prior (hence the head tossing and aggravation at the bit to start with) and desperately wanted to get away from that pain and her fight or flight kicked in.
I think the owner most likely wasn't honest about prior injury and had that been disclosed the whole situation could have been avoided. Big shame on her for hauling that poor mare to a show 10 days later, that mare is surely a saint for tolerating it.
I rehabbed a mare several years ago that had scars across her tongue from a similar deal where her tongue was all but severed. The owners brought her to me about a year or so afterwards as they couldn't do anything with her mouth (had to be tranq'd to deworm even), let alone put a bit in her mouth. It took quite awhile but I did eventually get her accepting of my hands in her mouth, still to deworm and riding in a rubber mouth snaffle. She was still ultra sensitive and any excess pressure (in her mind - mild for any other horse) would send her straight up. She'll never be able to be ridden with a bit in any but the quietest of hands. They now ride her in a rope halter and she packs their kids around, more than happy to work off seat & legs.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.