What should I do if a friend od's on cocaine?

Call 911. Assuming he has intoxication, or od on Cocaine this is serious, because of cardiovascular and neurological complications. Other concerns Cocaine can be cut with other substances and a simple od can be multisubstance in nature thus complicating the picture.
Call 911. They would need to be evaluated in the emergency room. Could cause seizure, heart attack, also could be a wake up call and a start to seeking help for the addiction, however this is often not addressed nor is it a priority of the emergency room. But none the less don't handle it on your own.