Why would coolant be coming out of the vent screw?

What is your question?

My son was driving the car; I found it with no fluid in the reservoir. After filling it with water and pressurizing the system with the cap. I cranked up the car and saw it beginning to overheat. I saw green coolant coming out of the vent screw. Why does this happen?

What makes this problem better or worse?

Not sure

How long has your 2003 BMW 330i had this problem?

Not sure; my son was the old driver