AutoCAD P&ID entities ByLayer?

I really don't know much about AutoCAD P&ID, so excuse me if this is a dumb question.

We have a designer creating P&ID drawings with the subject software, and the line work they create is NOT ByLayer.
We need it to be so we can control the layers (colors & line weights) with the (xref) layer dialog.

He tells me this is all built in to the software, some lines are on the same layer but with different properties, colors etc.
He has no idea how to control this, he uses it OOTB with tool palettes and minimal set up.

Is there some magic variable or setting I can tell him to change to fix this?

Any other info or advice would help.

Thanks