Full disclosure:
I ran an active business in SL from 2006-2009, then I gave up. I guess you could say I still run a business, but I haven't put much out in years and I don't even log in these days, though I get the bug once in a while and poke around.
However, when I started out back in 2006, I only made 200-1000L (linden) a month in the beginning. But within 6 months or so, I was making about $500 (dollars) a month and paying for all my shop rentals and land teir. I had a ton of vendor shops in nearly every niche mall. It would take me 8 hours to go around, once a month, and pay all my rent. To the OP: your friend that thinks he could work 4-6 hours and make that sort of money is wrong. I worked sometimes 14 hours a day.
At the peak of my income I was earning $1500-$2000 (again, dollars) a month. Then, within the space of a few months, it went from that to a few hundred, to 50 dollars, to almost nothing. I hadn't done anything differently, and the only thing I can think is that it was some combination of the economy crashing, big professional design teams moving into SL and the change of the search algorithm. And I knew a lot of other vendors at the time making more than I was. One person I know of made 'club' wear, and sexy dresses and made nearly $50k a year.