My impression was that the other officer was there, and was a witness. But even so, it does place that other officer in the awkward position that you describe. It has to be written up, or he puts himself at risk later.
It's tough. I really hope that the CM returns an acquittal if the facts are as reported. That's the right thing to do.
"Maynulet’s fellow officer, 1st Lt. Colin Cremin, testified that Maynulet told him he then shot the Iraqi in the base of the neck or the back of the head."