Leftseat.com shows 37 kidney transplants holding 1st class medicals.
https://www.leftseat.com/sistats.htm
FAA Guide for Medical Examiners:
https://www.faa.gov/about/office_org...se_prot/renal/
Old, and military-related, but on point:
One Kidney - Aviation Medicine - Baseops Forums
http://www.asams.org/guidelines/Comp...%20Disease.htm
Bottom line: decided on a case by case basis.