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.