Aw, geesh, I just typed a long post and lost it. I feel strongly enough about this topic I will re-write my response, LOL!
Any person can sue any other person for anything in any amount they choose. However, that does NOT mean they will win! I am not an attorney, and I am not providing legal advice, but I do have a knowledge of the judicial system.
The key to any defense is documentation, documentation, documentation. Verbal testimonies may never be heard.
I suggest your grandmother have the following documentation prepared for the judge's or jury's review:
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How long the boarding facility has been open (and nature and dates of any previous injuries, but don't provide unless petitioner brings before the court)
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Written testimonies from other boarders as to the good care their horses receive and the level of safety at the facility (you can go to ask.com for a format to follow)
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Written testimonies from others who have solicited your grandmother's assistance in healing horses
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Written testimony from the Vet
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A time log of the injury of the horse in question, the resulting phone calls to owner and vet, treatment by vet, and the daily care your grandmother provided afterwards
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Pictures of the facility and area where the injury occurred (as mentioned by a previous poster)
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Research and provide a written log of the petitioner's other law suits including the dates, nature of suit, and decisions (this may evidence the petitioner is sue happy)
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Written examples of how such a horse accident can occur involving no fault
Then I would suggest your grandmother follow the practices of all other facilities I have experienced in the following ways:
Lastly, my personal opinion is that your grandmother should charge much more for her monthly board, especially if she provides the hay and feeds. She should be making a profit with such liability. I've never heard of such low board, even though I know prices vary significantly through the country.
Your grandmother sounds like a wonderful person, so I hope she wins.
Perhaps she can petition the court for reimbursement from the petitioner of court and attorney fees if she wins. 
Your attorney should be providing this advice to you, but he would charge, LOL!
Good Luck!

No heaven can heaven be, if a horse isn't there to welcome me.