Yes, I also think your buddy is full of sh&! because I get 10 towing with my Chevy 6.0 if I'm lucky.
If you're looking at Fords in the 2005 age range you probably know about the 6.0 diesel. They pull really strong but there are certain things that are prone to expensive failures. For about $5K you can make modifications to make them reliable. Welcome to the new age of diesels, where if you delete this, modify that and monitor these other things carefully you can have a very powerful, economical and reliable truck. If you don't you may end up with a very powerful money pit.
With gas engines you sacrifice towing power and fuel mileage, but in return you get in the truck, turn the key and drive away without worrying about deleting your EGR or your DPF clogging with soot, or when you can sneak in your next regen.
If you can find a V10 truck in good shape, jump on it. The 5.4 will do the job but you may find it lacking for power, especially compared to your 6.4 diesel.