12 Valve no Power Bad Mileage

[drtfrmr]

I have a friend with a 98 12 valve 36000 miles. He has low power and is only getting around 13 miles to the gallon on the highway running 65 miles per hour. It has an automatic. I was looking for some solutions to give him. It has been to the dealer acouple times and to a local Diesel mechanic.