You could refer him to this, perhaps (my bold):
A length expressed in any of (cm, mm, in, pt and pc) will appear as exactly that size (within the precision of the hardware and software). They are not recommended for use on screen.
You could refer him to this, perhaps (my bold):
A length expressed in any of (cm, mm, in, pt and pc) will appear as exactly that size (within the precision of the hardware and software). They are not recommended for use on screen.