Well, since no one else has responded I'll give it my best shot. From the pic, it looks like he could have had a blockage or impaction of some sort. Did he feed him in his enclosure? And what kind of bedding was he keeping him on? He could have accidentally swallowed some bedding, or something else could have happened while he was digesting that could have blocked his intestines. I'm not sure though, and I could be completely wrong.

Also, I'm pretty sure I saw another post a couple months ago where someone mentioned that when they die their kidneys (?) turn greenish, so that could be it too. I tried searching for the post and couldn't find it, but I think it was something like that.

I'm sorry for your friend though It's always sad when an animal passes away, but when they die suddenly and for no apparent reason it's even harder.