Your friend seems to be having a simple renal cyst
. Renal cysts are generally an incidental finding on ultrasound
and normally no treatment is required for them.
Cysts are collection of water like fluid inside a sac.
Simple renal cysts are usually without any symptoms, however some cysts can give rise to pain in the abdomen on that side of the abdomen.
This pain is generally due to the adjacent organs getting pressed by the cyst.
If the cyst continue to cause pain, you may consult your doctor and get it either aspirated or removed surgically.