I owned a 2010 F-150 Crew Cab 6.5 bed and 5.4 with 3.73 gears and 4x4. My GVWR was 7,700 lbs. Sounds like he doesn't have the max tow package which gives you the upgraded cooling system package. I'm unsure if the regular tow package has the same cooling system upgrades, that's something you'd want to make sure of since you live in a generally warm area of the country. You can usually punch in the VIN of your truck in this web page http://services.forddirect.fordvehicles.com/inventory/WindowSticker.pdf?vin=1FT8W3BT3DEB19230
just change the VIN number after the "=" sign. This will give you a breakdown of what the truck is equipped with.
As for 1. The limitation on my max tow truck was actually the rims. Each rim was rated at only 2,100 lbs. The "P" rated tires were rated at 2,400 lbs a piece roughly. So I only have 4,200 lbs of capacity at each axle. However, there's a rag that noted a capacity of 4,500 lbs for the axle, so the rims were not limiting the axle by much
2. Air bags don't add capacity, they help level the load of your vehicle though so you're not dragging your rear end around.
3. For 3, it's the same as 2. I'll note again that in my case, it was my wheel rims being rated at 2,100 lbs a piece. The 9.75 axle (if your truck has that) can actually handle a little more, right around 4,500 lbs (can't remember, but some 4-wheeler mag did a comparison build between the 9.75 and the Dana 60). I can't say much for the other supporting bits though like the leaf pack and bolts that hold the rear suspension together.
Personally, as long as you aren't exceeding the axle weight of your rig, I'd be fine with it. Only thing to do is scale the setup and make sure you aren't exceeding the axle ratings.
