even if his chat window was in full view, the crafting windows would at least block most of the rest of the screen, so that if the GM appeared in front of him, he still wouldn't be able to see him. I know I've suffered this issue when crafting if someone walks up to me and starts chatting, I may not always immediately recognize it. I've lowered the transparency of the windows to help and deal with this issue.
Another thing, sometimes it's easier to adjust your camera at an angle where you have easy access to both the forge and anvil, so that your camera may not be facing immediately forward like a 1st person or 3rd person follow perspective, so if the GM appeared IN FRONT of the player, and attempted to communicate strictly through /tells, which may not have even been a tab on the player's screen, then I can easily see how the GM went unnoticed.