* Moved from Progress Reports to Process & Procedures *
1) Was he still married at the time the I-129F was filed? If so, this is a very significant issue.
2) He must disclose the past marriage on the DS-160. The CO will likely notice the discrepancy and request the divorce certificate. The only unknown is whether that is the end of it or if the CO will send the I-129F back to USCIS as it was incorrectly approved due to the incorrect information on the petition and lack of required documentation to go with that.
Edit: Technically a misrep bar could come into play, but I'm personally doubtful of that if disclosed voluntarily (while negligent in verifying the accuracy of the I-129F before signing and sending it, I would find it hard to believe somebody would claim it was willful if disclosed on their own).