BIG assumption that he doesn't need a sway bar. Some might say dangerous.
The trailer never decides to get squirrelly 5 miles from home. It's always the farthest from home, the farthest from help. Buddy of mine towed for years same truck and trailer, all of a sudden he's driving like a bat outta hades down I-81 and the trailer starts trying to pass him. Of course clueless macho man he taps the brakes, thinks nothing of it, and continues on his way. Imagine if it had scared the tar outta him like it should have, and he immediately pulled over and refused to drive any farther. Now what? Out in the middle of nowhere, far from home, far from help, nothing to fix the problem (there was nothing heavy to put in the front of the trailer). Cheap sway bar already installed would have prevented the problem in the first place.