If for some reason the FO had also selected the IRU1 source, then there was no chance of any warning being triggered when IRU1 started to go wild - displayed
The source select is a small rotary knob that take a firm grasp and rotation of 45 degrees from the normal position.
When using other than your primary source an amber ATT/HDG 1 or 2 is displayed across your PFD. It would have to be done intentionally and directed by the QRH handbook. That will have to be left to the investigators to determine.