First thing to know is that no 4-stroke engine runs better with back pressure.
This bit of information has been known since the 1890's, the first 4-stroke engines
So a few vehicle manufacturers have picked up on it in the last 128 years
On V6 or V8 engine you have the option of Dual or Single exhaust.
Neither changes the power output of the engine
You can add "H" or "X" pipes to dual exhaust to gain some Scavenged power
Scavenged power is used in all factory exhaust manifolds or headers, since the 1970's
It uses the Velocity of the exhaust leaving a cylinder to DROP the pressure at the other exhaust ports on that bank/side of the engine.
This drop in pressure means when the exhaust valve opens exhaust is PULLED from the cylinder which leaves more power on the crank, so more power for rear wheels.
This lower pressure can only be generated in one of 3 RPM ranges
Low-band
Mid-band
High-band
Rangers come with mid-band
From the exhaust manifold back as long as you don't restrict the exhaust flow there will be no change in the power.
Cats and mufflers don't effect power unless they are blocked, or the wrong size
Larger pipes from Cat back sound better, to some, but also won't effect power
Larger pipes on the heads will lose power
The "H" or "X" pipes on dual exhaust uses opposing Pulses to create a lower pressure in each pipe which PULLS exhaust out better, leaving more power on the crank
This ain't much but ain't 0 either, lol
Single exhaust is cheaper, thats why it is used
Dual exhaust can sound better and is good for advertising, but if it improves power then something was wrong with single exhaust system
If your friend wants more low end power then get headers tuned for low RPM band