Another way to look at it: Open differentials basically let the vehicle send power to the wheel that is EASIEST to turn on a particular axle. So hypthetically, if in 2WD mode on your transfer case, and the left wheel is really wedged against some rocks and the right wheel is on a sheet of ice, the right wheel will just spin and the left wheel will do nothing. This is what your friend means by "1WD". So, if you put your transfer case in 4WD and the same situation is happening with the front axle as well, you will only have 2 wheels that are spinning, hence this is what your friend means by "4WD = 2WD" if you don't have lockers.
My own friend once told me that you don't want limited slip diffs on a jeep, but I can't remember why.
following that, having one locker = 3WD
and if you have lockers on front and rear = true 4WD... but not very useful for turning.
Your friend is just being a smartass and trying to impress you. It is NOT like some go-carts that literally drive only one wheel all the time. Power can go to any wheel, it just depends on the circumstances.
Then, to complicate things even more, Jeeps use a brake lock differential. If the computer senses a wheel turning much faster than the others (indicating it's slipping) it will apply the brakes to that wheel and force the differential to turn the opposite wheel on that axle.
Works really well, until you NEED the wheels to spin...
To expand on the good explanations above...
Consider this. The open differential always splits the torque to both wheels 50/50.
On pavement, when you accelerate, both tires get equal torque, and they equally propel the vehicle forward.
With one tire on pavement, and one on ice, they still get equal torque.
The problem is, the one on ice may only take 50 ft/lbs to spin it. The other tire will get 50 ft/lbs, too, and that may not be enough to propel the vehicle forward.
Now, apply the brake to the spinning tire, so it takes 200 ft/lbs to spin it, and the other tire will get the same 200 ft/lbs. That may be enough the propel the vehicle forward.
An open differential allows a differential in speed.
An open differential always splits the torque equally.