You have probably heard of Claude Oscar Monet. He was one of the founding fathers of an art movement called impressionism. To Monet, and the other impressionists, the fine details of an image were not important: their focus was on using light, shade and colour to leave an impression of what was being depicted. Prints of Monet's paintings, particularly his series of paintings of water lilies, have graced many walls in recent years, and continue to sell and be produced in large numbers. In the early years of impressionism, however, the art establishment considered his work worthless.
When Damien Hirst pickled a shark in a large fish tank, I found it repulsive. I still find it repulsive. That's the nature of art: not everyone will like it. If it doesn't provoke a reaction, either positive or negative, can it really be called art? My own preference is for monochrome line art, or maybe duotone comic art; actually, I love duotone. If I could find some duotone art to hang on my walls, I would be a very happy man.
http://valiantonov.deviantart.com/art/white-queen-Duotone-289579745 and, once again, kudos to the artist. The subject matter is not to my taste, but it gives you some idea of what I consider to be the pinnacle of artistic achievement. You will disagree with me, maybe, and that is the whole point. Art is subjective - I like duotone art, whereas you may like full colour paintings, and you may argue that watercolours are superior to oils, and others may take the contrary view, or say they prefer sculpture as an art form.
I know that I went off on a tangent there, but I did not stray from the point I am trying to make. My friend may use spray cans and concrete (and other surfaces), rather than oils and canvas, and that means some can not accept it as art. It challenges them, just as every art movement in history has challenged critics.
Do I consider graffiti to be art? I could be accused of bias here, so I will define what art means to me. Wherever you are when you experience a piece of art, it takes you somewhere else. It may be a piece of poetry or a book you read; it may be a piece of music; you may find it in a painting or sculpture. The very best art takes you to a place, within your imagination, that is outside of your everyday experience. Would I spend my time looking at a bare concrete wall? No. What about the random acts of vandalism committed through the medium of the spray can? No, they are very much an unwelcome part of the environment I live in.
If you consider a bare concrete wall to be attractive, I won't argue with you, because it is your right to have a preference, but maybe you should seek some form of therapy. If you don't like street art, that is also your right, but no one asked you to hang it in your home. Some of us see a blank space filled with something that brings a smile to our face, or makes us feel admiration for the work that has gone into the piece. To each their own, I guess, but there is a long history of controversy generating fame, so maybe my friend should not be offended by those who don't agree with what he does, but should welcome the fact that his work is being discussed. His work pushes the boundaries and challenges the popular conception of art: that it should hang in a gallery for the privileged few to view.
Now, please excuse me while I search for a supplier of framed duotone comic art...
Update: An article on the ship which my friend, Andy "Dime One" Birch was involved in painting - http://www.leaderlive.co.uk/news/115872/mystery-of-llanerch-y-mor-funship-graffiti-solved.aspx