It has always amazed me how many people "accidentally" shoot somebody while supposedly cleaning their gun, since one of the parts you clean is the part where the ammo goes waiting to be fired. As common as this seems to be, maybe how to unload a gun before cleaning it should be part of the CHP classes.
Maybe he was trying to run a cleaning rod into a cylinder with a round in it (and he's blind so he couldn't see it) and the cleaning rod, which was a special type of cleaning rod with a small steel end on it, dented the primer, deep enough to fire the round. The cylinder was swung out of the gun, but the bullet still had enough force behind it to injure the officer, even though it didn't go through the barrel. I think this is about as likely as any other story about a gun going off while it was being cleaned.
My guess he was playing with the gun, probably pointed it at his friend and pulled the trigger assuming it to be empty. Or maybe there was an argument and a scuffle that escalated too far, and after the officer was shot they both decided it would be best to make up a story. You'd think they could come up with something better than the overused gun cleaning story though.
That's all speculation of course. My guess is we'll never know what really happened.