Here are some off-the-cuff ideas that might help:
(1) The ONLY way (aside from inheritance/luck) to get wealthy in a free market society is to produce goods or provide services that others demand (i.e. need or want). Challenge your friend to DEMONSTRATE A CAUSAL CONNECTION between one person accumulating wealth and another person subsequently falling into poverty. There is none to be found, since none exists. He may produce numbers pointing to increasing poverty rates, but those will be bogus because “poverty” changes in definition, and relative to the times, “poor” people today are way better off (materialistically) than rich people in the 18th century.
(2) I think you have too narrowly framed “the lesson.” I know it was not your intention to give the book definition of it, but just want to make sure it’s clear. Hazlitt’s point is that there are two economic fallacies commonly committed, which lead to negative outcomes The first is that people ONLY look at short-run consequences rather than long run consequences. The second is that people ONLY look at the effects of policy on specific group(s) rather than the effects for all groups.
(3) I honestly am not familiar with the term “closed economic system.” (I am quite new to liberty myself). Perhaps these remarks are relevant to the issue. We do NOT live in a system where money given to one party has been TAKEN from another. You are correct that the key is there is no forceful transfer of goods on a free market. Exchange on a free market is VOLUNTARY which necessarily means that both parties benefit from the exchange. It is not the exploitation of another man to charge him $25 for sneakers; it is the wonderful benefit of a system where the man may spend as he wishes.