Well it's only logical that that wouldn't work;
The relationship is based on the ID field but when you remove the ID field that you brake your relationship.
If he wants he can show the company name in stead of the ID in the dropdown and once he created the selection.
This file is an example that shows how this works:
I tried posting the link to this thread on the youtube comments but got an error.
I don't know if this person knows how to get here?