It sounds like your friend’s real objection to capitalism is that there seems to be no organized structure for planning. Marx called it “market anarchism.”
There are a couple of steps to take. But first you should challenge the idea that the failure of socialism is the fault of its leaders, and not the system, by pointing out that it is the nature of totalitarian governments for corrupt people to rise to the top in those organizations–just as in organized crime.
Hayek made this point in “The Road to Serfdom” nearly 60 years ago. This is a little refresher.
To the main point, the two best arguments are the lack of incentives and the inability to calculate costs in a state-run economy, which became very popular after Ludwig von Mises published the idea. (I don’t believe he came up with the theory first.)
The first one is rather easy to understand. Marx discusses this. He counters that a capitalist-free people will make goods so abundantly that the problem will be come mute and dissolve away. In other words, he completely dodges the issue.
The second point takes some time. First, capitalist economies thrive partly because information is more easily understandable and readily available–the main focus here is prices and profits.
Both give information as to what is demanded, how well it is served, as well as how is best to provide the product, if at all, and tons more.
Further, socialist governments are unable to know the opportunity costs of their actions because there are no prices once money is outlawed. Money provides for common “language” to speak in terms of. In socialist nations, you are literally comparing apples and oranges, with nothing meaningful in common to compare them to.
Also, with no private property, any money that did exist would be useless since it is the state that would be both buying and selling the product inputs, rendering the transaction meaningless. It would be like taking money from your left pocket and putting in your right.
It is utterly ridiculous to claim a particular economy is superior if it is unable to calculate costs of an act. It is even more surprising someone would be using that line anyhow; most focus is on more egalitarian motives to support the various branches of collectivism.
Then there’s the issue of the division of labor, commerce promoting peace, balance of economic powers; the list goes on.
I like the following articles myself:
Proof That Socialism Cannot Work
Socialism vs. Market Exchange
National Socialism