There is no requirement I know of for a PE to "certify" the weld quality. This is normally done by a structural testing firm specially engaged to perform a "SPECIAL INSPECTION", but only when it is required by the engineer of record or local jurisdicction.
Questions: Did the contractor have a current welding certificate with the proper qualifications to make the type of weld seen? Was he licensed and bonded to do this? What type of weld was it? A fillet, or full pen butt weld with backing bars? What electrode was used and what steel? Where in the beam was the weld made? There a lot of questions here. He should be able to answer all of them.
Please do not be offended, but for anyone to blindly hold an engineer responsible for the actions of a contractor, who, in all likelihood, did not want the engineer involved as he would complicate the "solution", is ludicrous in my mind.
Yes, I am reading between the lines here, but speaking from experience. Not admissable in court, but an indication of where to look harder for that admissable evidence. Just my two cents.
Mike McCann
MMC Engineering