You can't become a billionaire through honest earnings. The degree is variable but the presence is inevitable, becoming required more the more billions you amass. Of course, you can take the ethical route, but those people don't get the billions.
I know a billionaire who basically was in the right place at the right time during the biotech boom. He started a company as a side project (he was a professor), got a deal to supply a major pharmaceutical company, and expanded from there. I don't know every decision he ever made but I'm not aware of any unethical behavior. He lives fairly frugally and has made a point of staying under the radar (he's successfully avoided being put on Forbes's list of billionaires, for example).
I'm willing to argue that it is in fact possible to become a billionaire though honest earnings.
I probably shouldn't make such absolute statements, they do beg for counterexamples. I'd be surprised if your friend didn't engage in some anti-competitive or employee-exploiting practices, but of course I have no idea.
'CrazyStat answered the ethics part, but I also want to point out that "earning to give" doesn't mean you have to become a billionaire. It means you want to maximize your earnings to the degree you're capable and comfortable with ethically. "Ends justify the means" is not a part of "earning to give" philosophy, even though some people probably adopt this principle.