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 (click to enlarge).
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 angle on the lens. When light hits a surface two things happen (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). Every time the light hits the surface of the lens, a part is reflected and another part is transmitted. But the lens has a thickness so when the light that was first transmitted now hits the back of the lens, a part is transmitted to the CCD sensor of your camera and a (smaller) part is reflected back into the lens. This can happen several times. Normally this effect is small enough it will not be noticed. But this picture is not a normal case. Most of the picture (dark sky) emits no light, the picture is very dark. In contrast to this, there is a bright light source dominating everything (related to luminescence). And we have a 7 sec exposure the picture is taken with. 7 sec are enough time for the CCD sensor to capture some light from the effect described above.
If you now want to know why is it green and not white. White light includes all wave lengths (colors) and every wave length is reflected in another angle on the surface of air-lens. The green wave length just had the best angle to be reflected severals 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.