I can't see how he could be within the vehicle-specified 500 lbs hitch weight limit with that TT. Not only is it going to be light on the steering, it will also be at risk of tearing the receiver off the car. Ask him how he'd feel if his TT rips away and careens into oncoming traffic at 60 mph. Subaru saying to not use WD could indicate that the sheet metal where the receiver is mounted can't take the strain, so what will overloading the receiver do?
And trying to load more stuff in the rear of the TT to compensate will only make the handling squirrely, risking sway.
He's got about 700 lbs of humans plus dogs in there, plus the hitch weight, plus who knows what else... the rear suspension will be hard pressed, as will the rear tires.
Offer to take that TT to the scale and weigh it for him, maybe?