I would definitely point out, early on in the paper, that the victors of history tend to write it, and that this usually ends up skewing many events that have occurred. The Great Depression is no exception; nay, it might be the poster-child for such revisionism and white-washing. Worse, still, it’s being taught in government schools without any radically different views being applied. If your lucky, he may agree with you concerning going back to a gold backed dollar, but I doubt he’ll relent unless you give a through analysis. Even so, I doubt he’ll abandon his views of central banking.
As for your question, I would point out that what they were investing in isn’t so much as important as the fact that a recession occurs naturally as a result of unsound investments. A recession is an important part of a cycle that allows for people to re-analyze their investments & strategies, which leads to corrections, which leads to the market correcting itself and life goes on. I would point also dig up examples of ‘scares’ that happened in the past that would’ve been on par with the great depression if there were no Federal Reserve established about 17 years before the great depression hit.
However, I’m just really kind of dipping my toe into the economic waters; I’m sure someone else could give a better explanation and/or a more correct explanation.
I would however, mention this site and/or use it for research: http://www.dailyreckoning.com/index.html, as it gives fairly detailed info. It might seem like a more official source to your teacher as well, which sadly, might matter more instead of correct logic.
I would also, since you’re dealing with a Leftist, try to use more emotional arguments for the consequences of central banking, verses laissez fare. I remember doing that a few times myself in one of my history classes in discussions, and while my economics knowledge was vague, I still got a pretty good response from my professor for being able to question the common conceptions of central banking, despite herself admiting she was a progressive.
Also, I would recommend reading anything on the recommended reading list concerning economics; I myself have been skimming through De Soto’s Money, Bank Credit, and Economic Cycles (http://mises.org/books/desoto.pdf), as of late.