Thank you for pointing out this fallacious argument. An objectivist should be able to prove this, however history and the economic principles that are found thereby show that while government is often a very significant factor in the generation of a monopoly, a monopoly is a "cornering" of a market which can and does happen by market forces combined with human greed in laissez faire markets as well as government controlled markets.
There are many simulations which demonstrate this. One of them is made by Milton Bradley.
If you believe a monopoly is evil only because there is one company alone providing a service or product, please explain why that is evil. On the other hand, please name one monopoly that was able to control or "corner" the market by market forces alone that did not do so by offering the best product at the best price (in which casw it was benevolent) or, if coercive, without the influence of government force. Just name one.