Hey,
Well, let me start by asking you if you've taken the snake to the vet, and whether they have suggested to you that there is nothing wrong with it. You said something about if "this is something that the vets are not familiar with". I guess that was a rhetorical point. At any rate, with these cases of malnourishment or under nourishment, it is possible that these animals are suffering from some kind of an internal ailment secondary to a parasitic infection.
What throws off the ability to hold the food down is, the abnormally high bacterial load in the gut, that is usually held within acceptible levels in an otherwise healthy animal. To return the gut-flora to a possitive homeostasis, it is usually recommended, that, the animal or animals in question be treated by a vet or someone with experience with this type of ailment. If, such an ailment is ruled in - parasites and abnormally high bacterial load, and is found to be the cause, the animal should be treated with the appropriate medications. Flagyl and Metronidozole (Panacur), could possibly remedy the cause for the regurgitations. But, the meds must be administered by a vet the first time around and treatment must be followed up by several doses thereafter, by you. A fecal examine is warranted and should be examined by an experienced vet, accustomed to treating reptiles, and one that understands reptile ailments.
If you get the chance to collect fresh fecal material from the snake, you should take that into a vet to perform a fecal examine. These are only suggestions as to what the possible cause can be -- there are other possible causes and agents that can cause regurgitations and distress in a snake.
If the snake was housed communally with other snakes, while in the care of the original owner, it is possible that the other snakes too, were infected with a similar malady. Vectors for transimission of either parasites or bacteria, take on various forms, including bad husbandry. Fowl water, recycled food, not eaten by one snake and subsequently offered by the keeper, to another, can contribute to disease. Saliva, feces, mites and other insects, such as flies and roaches and other vermin, can all participate or act as vectors for disease. Look into the first option and get a fecal examine of the snake done, along with a possible treatment for the snake started as soon as possible.
-Angel
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Discovered and defeated of your prey, You skulked behind the fence, and sneaked away. --Dryden.