What is wrong with the idea when Ben says, "Never fight fair with a stranger. You'll never get out of the jungle that way".It is on page 49

I suspect that this provides an alternative to Willie's theory of life.  Willie believes that being well liked, personality, is the key to success in life.  Biff sort of play fights with Ben (depending on the production you see); when he finishes, Ben drops him and explains, by example, that it's not "niceness" that gets you ahead in this world, but the exercise of "power."

If the business world is a "jungle," then Ben presents the case that the exercise of power, even ruthlesness perhaps, is the way to get ahead; it's a play on the old saying that "nice guys finish last."

I'm not sure there's anything wrong with it ... unless you're a Willie.