I agree that guy has a major problem I hope he figures out before it puts him upside down or kills someone. But I donโt believe merely throwing a WDH or anti-sway bars at it is the universal solution.
Before applying those, one ought to make sure all the other parameters are right, among them rig within specs, tongue weight right, tire pressures on all tire correct. If all that stuff is right then a WDH and/or sway bars can be helpful. If they arenโt right, WDH and bars are a duct tape fix, and while duct tape can patch some things, contrary to popular opinion, it isnโt a cure all.
BTW, full disclosure: I tow a TT with an SUV. No WDH, no anti-sway bars. It tows straight as an arrow, never sways, no wander in the winds, and no reason to add those things.