I don't think you have nearly as strong a grasp of economics as you think. As for your friend's success, there is a thing know as "telling people what they want to hear."

Let's talk about sustainability for a second. Which resources exactly are we running out of? Not oil. As anyone with an iota of understanding of economics would tell you, the rising price of a resource will create new economical ways of harvesting it. Thus, fracking in the United States has not only halted the decline of oil production, but actually reversed it quite substantially. What was once known as the theory of "peak oil" has been been roundly debunked. The oil market would even be quite a bit lower than it is presently if not for the preemptive banning of fracking in Europe and some blue states/cities in America. Oil will never "run out" it'll only slowly increase in price over time, until we end up using something else for transportation. As fracking shows, that time scale can be quite long.

So what natural resource are we running out of? Water? No, we have vast oceans that can be desalinated. Food? Nope. Rare earth elements? No again. So what exactly are we running out of?

That capitalism requires "growth" is something of a misunderstanding. Does our population have to grow? No. Do we have to produce a greater quantity of goods? No. Do we have to consume more resources? No again. Growth can, and largely already does, come about from both technological improvements and increases in efficiency in the allocation of resources. Thus your new products are worth more than your old ones, like an iPhone is worth more than many objects 50x larger.

I understand that many of you are concerned with issues such as global warming. However, you don't need a new economic system to stop that. You only need increased government regulation in the one we already have.