Line 7 of the W-4 sets forth the two criteria which must be met in order to claim an exemption from withholding:
"I claim exemption from withholding for 2017, and I certify that I meet both of the following conditions for exemption.
- Last year I had a right to a refund of all federal income tax withheld because I had no tax liability, and
- This year I expect a refund of all federal income tax withheld because I expect to have no tax liability."
So it's not enough that he got "most" of his withholding back last year; he would need to have received all of it back.