In your friends picture are more artifacts than the one you showed in the 2nd picture a little bigger. I marked more of them in the picture below.
They are all in a perfect line to the bright light. So these artifacts are caused by the bright light and the lens of the camera. A lens is not flat it's, well lenticular (it's where the name came from). The main light source is shining from an angel on the lens. When light hits a surface two things happens (at least in most cases ignoring Brewster's angle). The two things are reflection and transmission (or absorption, but we are talking about a lens so i let this one drop here). Everytime the light hits the surface of the lens a part is reflected and an other part is transmittet. But the lens has a thickness so when the light that was first transmittet now hits the back if the lens a part is transmittet to the CCD-Sensor of your Camera and a (smaler) part is reflected back into the lens. This can happen severel times. Normaly this effect is small enouth it will not be noticed. But this picture is not a normal case. Most of the picture (dark sky) emitts no light, the picture is very dark. In kontrast to this there is a brigth light source domenating everything (related to luminescence). And whe have a 7 sec exposure the picture is taken with. 7 sec are enougth time for the CCD-Sensore to capure some light from the effect descriped above.
If you now want to know why is it green and know white. White light includes all wave lengths (colors) and every wave length is reflected in an other angel on the surface of air-lens. The green wave length just had the best angel to be reflected severel time and still reach the CCD-Sensor.
So the most accurate answer to your question „what is this green thing“ is: It's the flashlight (or what ever this light source is) that is seen in the bottom of the picture.
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