I think this all depends on the type of niche you're in. For instance, if you're into loans, insurance, lawyers, mesothelioma etc. in those type of niches advertisers can pay up to $60 or more per click.
In the lawsuit niche, advertisers can pay up to $98 per click. For college student loans, advertisers pay $70 per click. So what you're asking really depends on the niche. People fail in adsense simply because they go into a niche where people are paying pennies per click then expect to make a lot of money.
The key is to choose a niche and keywords wisely. The point is, affiliate marketing is not better than adsense and vice versa. None is better than the other, you can still fail in affiliate marketing if you choose a niche where you'll get low commissions, for instance.
Your friend did a good job by split testing to find out which one is working better for his website. That's what all marketers should do. So always test to see which one works best for you. If you want to use adsense then make sure that advertisers are paying a lot of cash per click in the niche you're targeting.