750 has 2 parts: 750A and 750B
ETA750B or 9089 is REQUIRED for NIW 140. It is a very simple form where you just fill in some basic information of yourself. 750B or 9089 does NOT need employer's signature. You can DIY.
For exmployment-based 140, both 750A and 750B are required. 750A is the form which needs to be filled out and signed by the employer. And it needs to be certified by the DOL.
He received RFE because he did not include 750B or 9089 in his NIW application.