A voluntary departure order, at least in Canada, is given to someone who is in Canada illegally and has been ordered to leave, voluntarily. As long as they comply with the order, and get it signed off when they leave, it's not the same thing as a deportation and they don't need authorization from the Minister to come back to Canada. I don't really know if a voluntary departure order is the same thing in the States, but it kinda sounds like it. What I've never heard of before, though, is such a situation ending up showing up on an FBI criminal report. I do know that ALL arrests show up on FBI arrest reports (criminal clearances) - whether they are eventually dismissed in the Courts or not. So - apparently - what happened at the border was considered an arrest, even though he was never convicted of any charges. This is going to have to be disclosed to CIC - and they will want all of the information/documentation pertaining to the disposition of the case. Immigration offences are not criminal offences - but it is a serious issue, I think anyway, simply because it shows up on the FBI.
If it were me, I'd do two things: get a copy of the FBI so you can see it for yourself. And then take the report to a reputable Canadian immigration attorney for an opinion on how to best address it in the application. I don't think, though, that it will make him inadmissible - but that's just an unprofessional opinion that's not really based in fact or experience. As I said, I've never seen this situation come up before. All I know is that there have been people who've complied with voluntary departure orders from Canada and they aren't found inadmissible for Canadian PR because of it.