lbf can be one of two things, neither of which is tractive effort. There's the English force definition, which is tied to acceleration (https://en.wikipedia.org/wiki/Pound_(force)) and the equally confusing lbf
of torque, which used to be stated as foot pounds. Why some eggheads decided to change foot-pounds of torque to pound-feet (lbf) and make it more confusing is beyond me. Your friend may have taken it as torque and figured it against a driver diameter or some such and come up with the incorrect answer. Earl and Bruce have the tractive effort formula down, which is also not to be confused with Draw Bar Horsepower.