Ooops!
Regardless of my number mistake...His 5th pin on the trailer is most likely for the braking system (a trailer with surge brakes would only have a flat 4 in my experience).
He needs to get an adapter, or rewire one on his truck, to acknowledge the "blue wire/brakes" (5th Pin) that actuates the electric reverse solenoid on the trailer. Otherwise it will not disengage.
I think you are right about the blue wire.