Epakitin is a phosphorus binder. From what reading I've done, using a phosphorus binder is recommended in chronic kidney disease when phosphorus levels increase as a result of the disease's progression.
What was Bud's Creatinine level? A BUN of 36 suggests typical diabetes caused dehydration to me. Creatinine is the more important kidney function value in addition to a urine specific gravity test to see how well the kidneys are concentrating urine.
Unless her phosphorus and creatinine were high, I don't understand the need for Epakitin at the stage of "renal insufficiency." Gandalf had renal insufficiency for the last 3 years and he maintained his values on sub-Q fluids, he did not have kidney disease, just overworked kidneys from the diabetes. This is my opinion, but without the high values to support the need for a medication, I don't understand why the vet would want to use it.