I actually asked him that at the time

Oh, is it a braked trailer?

No.

In that exact case yes, but I like to operate on the assumption that my trailer brakes are permanently broken. I've had them fail to operate mid-trip because they're fucking trailer brakes.

I am pretty sure physics says "no":

Ff <= µ*Fn

• Fn is proportional to the mass of the trailer, so it is irrelevant for the deceleration (a) if it is empty or not (because Ff = m*a).
• µ is a constant relying on the tire/ground connection. Comparable to the towing car (if you happen to have car tire quality ones on your trailer, which might be doubtful)
  .

On top of that, typical car trailers use overrun brakes that per definition work by exerting an additional force on the towing vehicle in the direction of movement that is then used to activate the trailer brake.