Deffo see if your friend will call the vet. I would be less likely to think she had a blockage and more likely to think she has liver issues.
For the liver the main red flags are sudden extreme weight loss, lethargy, dull coat, pale, not interested in feed.
If yet he horse has a blockage she would be showing some signs of impending colic, her droppings would be a telltale sign.
The vet would know which tests are best suited to her.
Please let us know how she gets on and good luck x
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