I have a friend that has gout and is worried about the medication side effects, does anyone know the negative effects of allopurinol?

Liver. It is common practice to check liver function while on Allopurinol therapy.
What kidneys. If on Allopurinol you should have your kidney function check periodically.
Why doesn't . Your friend discuss this with the prescribing doctor? All risks, benefits and side effects should be discussed when prescribing a medication.