Can my friends pull my 5th wheel down to Mexico/ Some one told them it had to be the same registratin as the vehicle pulling it??

Secondery insp.not same lic. Plate

They will likely send the tow vehicle and 5th wheel through secondary inspection, where yes, they will ask to see your registration for both. If the 5th wheel's registration is not under their name and you are not with them they might not let you through. This is highly variable though because sometimes they dont even bother with secondary when travelling South.