Only sure way to check kidneys is to run tests. Dehydration will prevent a snake from not having much liquid with bowel movements. However reptiles do not expel urine like we do (and other mammals), they expel urates, which look basically like a hunk of chalk (though it may be discolored by fecal matter).
As this ball isn't eating much on its own, its not surprising its not passing much out the other end either. I would double check its husbandry, make sure the temps are right, humidity is good, appropriate hides etc. Number one reasons balls don't eat on their own is they are stressed. Force feeding (forcing food down the snake's throat) is highly stressful. Assist feeding (basically putting the head of a rodent into the snake's mouth, then leaving it swallow it on its own) is a little less stressful.
Offering live rat pups (or smaller, depending on the snake's size) can also entice it to eat on its own. Do not leave the rat pup (basically a weaned rat) alone with the ball python for more than an hour. Or try an African Soft Furred rat or gerbil (though pre killed be best, gerbils seem more inclined to attack than rats).
If none of the above solves the problem, tell your friend to take their snake to the vet.
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PHLdyPayne