Hi guys,
Update! contacted deferred inspection site via email . Explained the issue and got dad's I94 updated to current passport. Also got written confirmation that stamp in passport doesn't matter, the online record is what matters . Here is the response verbatim " It does not matter if the stamp is correct or not. All that matters is the online record, which is correct." I hope this helps others who may be in a similar situation. Since I have a written confirmation from an official CBP email address, my parent will stay the extra month!
Appreciate everyone's advice here!