This should be a moot point.  Is music not included within Schools of Fine Art?  Artistic expression finds its form in architecture, visual arts, and music.  All are creative acts from the minds and souls of individuals, not from any other source.

If you want to be technical about it, music is written down.  It is no different than a poem or beautiful painting.  Yes, one has to be able to read music in order to appreciate its written form, but the same could be said of many other written arts.  Even a painting requires a certain knowledge set to be trully appreciated.  If art is "the quality, production, expression, or realm, according to aesthetic principles, of what is beautiful, appealing, or of more than ordinary significance," then we must conclude that music and dance fall into this category as well.

I would say music is a form of art.  We usually call it a performance art.  Music is a form of self-expression, and that uses an artistic sense.  Art does not have to be visual.  Art is creativity and vision.

Musicians who perform and chronicle their work are called "musical artists," and I agree that music is a true art form. However, there is a big difference between someone who perfects their trade, masters their instrument and creates an endearing piece of music and a person who does not. Only the finest musicians, singers, lyricists and composers can be truly called an "artist."

I have to agree with the previous post.  I can't think of any reason to say that music is not art.  It is, after all, produced by people as a way of conveying ideas and emotions to other people.  I would say that this makes it an art.

Visual art/"fine art" is just one kind of art. "Art" is conventionally understood to include many modes of expression, from music and sculpture to dance and photography. 

Constraining a definition of art to hand-made visual art would exclude many of the most popular forms of expression from consideration as art: literature, theater, photograpy and music. 

I would say that your friend has a semantic issue with the question at hand. My guess is that she takes the term art more narrowly than most and I would argue that this narrow view is both technically and conventionally incorrect. Aesthetically-oriented expression is certainly not limited to visual art. 

Art... 4.a : the conscious use of skill and creative imagination especially in the production of aesthetic objects; also : works so produced