WTF?
The author’s sales of sheet music to everyone who buys it include an explicit contract (called the laws of whatever country the sale is done in). The girl seems to have gotten a copy from some party to that contract, explicitly against its terms, and is now re-trading the item in question (the sheet music file), on her own. Except the law, as written, quite clearly applies just as much to her as it does to the person who violated his contract with the author in providing her the sheet music.
The nature of the stuff being traded makes it somewhat different than physical goods, but in principle this is no different any other illegal redistribution of goods, for example reprinting cheap copies of some copyrighted book and selling them on the street at barely above cost.
When you talk about your “ethical perspective as a libertarian”, what you apparently mean is that you don’t believe in the concept of copyright (or other “intellectual property”, or perhaps even that society’s laws should be binding on citizens). This is not the usual libertarian “ethical perspective”, and you should be careful about implying that libertarians necessarily feel the same way you do.
> successfully earn in the market
There is only a market for sheet music insofar as that sheet music can’t be trivially gotten for free from a third party. Basically, your comment says something like “we should eliminate the market for sheet music, and the author should do something else.” Well okay, that might be what happens anyway, but that’s not “the market” deciding. For markets to exist requires a set of social structures which regulate its form and function. When those structures break down, as in this case, the market is destroyed.
(These social constraints need not be written down or enforced by an official police force, etc. They just need to be commonly agreed upon and have some social weight. I need to know you won’t take my money and run without giving me the promised chicken, or spend all your time copying the book I sold you and selling it behind my back while I’m trying to write the next one.)