At 55 mph the assist wheel is spinning at approximately 2300 +- rpm so the wheel bearing must be stout.
I'm going to give the guy a little credit, and say this was just a temp extension rig to get the boat in the water. He probably needs the extra length to get the boat in the water, and the wheel keeps the bar from digging in. We had a sail boat where we needed to attach an extra 8 foot of trailer tongue so the Suburban didn't drown at the ramp. This thing actually looks decently thought out. I'd bet that hitch riser gets attached directly to the back of the car for road use. The rest of these setups - sketchy.
And, my favorite: I wish I could find the other pictures of this. They used self-tapping screws to attach the hitch to the tailgate, swing-set chains to help hold up the tailgate, then whittled a chunk of log to go between the receiver hitch stinger and the underside of the tailgate.