If the fuse didn't blow when hooked to his buddys trailer and did 3 times when hooked to his,then he has already isolated the problem to his trailer.
As posted above,I would start with a look in then trailer junction box usually these are just back from the hitch under the frame,some are on the A frame its self.
There is an obvious direct short,meaning a hot wire is touching the frame or a ground wire somewhere.
This could be so many things,a screw run through a wire,a wire striped where it can touch the frame or aluminum skin,a bad clearance light or tail light,or since this is a new trailer it may have been wired wrong in one spot.
Again,start in the juntion box for the obvious ,disconnect the tail light wire from the harness plug the harness into the truck if it dosn't blow a fuse then you know your good to the juction box. then go back to the tail light wire and use an ohm meter touch it to the wire and to the frame,if you get a reading or beep depending on the tester that will tell you you have a short some where in the trailer, then start looking at fixtures.I always solve this stuff by a process of elimination.
2010 298 RLS Eagle
99 Ford F-250
1 Toy Fox Teror
1 Tiny Chi Chi Wa that rules all of the above.