Hmmm... hard to say. It certainly could be the memory misbehaving, although when I've run into this in the past, it hasn't been hardware related. You could try removing that RAM stick to see if it boots without it (assuming there's still enough memory for it to boot).
Two other tests come to mind that should resolve things:
1. Bring his screens/keyboards to your place and try them out.
2. If they work, bring your screen/keyboard to his place to try them out.
There may be something with just where it's set up... and yeah, I'm really grasping at straws now
