Hello Sos19528,
A ruptured achilles tendon should not cause an elevation in
creatinine or a decrease in kidney function.
Regarding the question about urine output, this can be deceiving. In some cases the urine output will decrease; however, in other cases the kidney looses its ability to process fluid so the kidney may not be able to reabsorb fluid that is filtered by passive flow. In this case, urine output may be normal or even abnormally high.
Further testing is definitely important to find out why your mother's kidneys are having trouble.
Best wishes.
Advice and opinions given are based on the information provided and must not be used as an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.