Are you wondering if it's legal to ride across on a 3wheeler itself or simply crossing via a remote location on ATV?
He can cross remotely provided he gets a permit in advance from the CBSA. It is called a Remote Area Border Crossing permit and costs $30.
Keep in mind he also must meet US border crossing laws when he returns, which is phoning in to US Customs when he crosses back, assuming he was issued a RABC by Canada in the first place.