Was he issued any paperwork when he was "denied"? If so, the paperwork will reflect his status. If he was formally denied and found to be inadmissable, then under most circumstances he will have received a lifetime ban from utilising the visa waiver program and this will be noted on the refusal paperwork that he was issued.

If he was simply permitted to formally or informally "withdraw application for admission", then he may still be eligible to use the VWP. However, keep in mind that there is almost certainly a record of his previous interaction and this will result in his being referred to secondary at his US POE where a more in-depth analysis of his admissability will be made.