Who does "she" refer to in "she is like most artists" in "The Nightingale and the Rose" by Oscar Wilde?

In the passage you reference, "she" refers to the Nightingale. Here's the passage in full:

“She has form,” he said to himself, as he walked away through the grove, “that cannot be denied her; but has she got feeling? I am afraid not. In fact, she is like most artists; she is all style, without any sincerity. She would not sacrifice herself for others. She thinks merely of music, and everybody knows that the arts are selfish. Still, it must be admitted that she has some beautiful notes in her voice. What a pity it is that they do not mean anything, or do any practical good.”

In the story, the young student cannot understand what the nightingale is saying to him; he only knows what he reads in books, and the language of the nightingale's song appears strange to him. While professing to be a student of logic, he is blind to the logic of true self-sacrifice.
In reality, the nightingale has already consented to sacrifice her life's blood for him, but the young student is unaware of this. He accuses the nightingale of being selfish, but it is he who lacks true discernment. What he considers as the vain song of a bird is in reality a song of self-sacrifice. All the nightingale asks in exchange for forfeiting her life is that the young student will be a "true lover."
Ironically, the young student can appreciate the "beautiful notes" in the nightingale's song but fails to recognize the "feeling" and "sincerity" in her anthem of love.