I would say that he is going to need proof of ownership of the ATV's, and trailer if that is what he is hauling them on. That along with whatever your area requires of a non resident. If his ATV's are not registered where he lives in the US then I would say that he should have a 4457 forum filled out by the US Customs office before he crosses over from the US, that form is proof of ownership, the 4457 wouldn't be a bad idea even if they are registered.
I personally wouldn't bring anything into a different country without knowing exactly what is required by me so that I could bring it home. If he really wants to ride a ATV with you I would suggest a rental in your area.