For me, what it mostly comes down to is:
1) What's the cargo carrying capacity sticker say?
2) If he hits a scale, what is his GVWR (gross vehicle weight rating), RAWR (rear axle weight rating) and FAWR (front axle weight rating) compared against his actual weights?
3) What is the tire load capacity on the tires + wheels he has now?
Edit to add: obviously I didn't read your original post closely enough. You've answered a lot of these.
I agree, with a GVWR of 6,700 pounds - he's going to be hard-pressed to tow much of anything once he adds the rest of himself (if over 150 pounds), passenger(s), pet(s), stuff into/onto the truck, hitch and have anything left over for pin weight.