Even if they want the physical alterations, how is it any different than someone without a gender identity disorder who wants to enhance their apperance through surgery? Are they hurting you in any way by wanting to live as the opposite gender? Are they hurting someone else? If they are happy with their decision, let it be their decision.
I have learned a lot over the last 8 years since my brother told us that he wanted to be a woman (not nearly enough, in my own opinion, but more than many people, especially those who don't personally know anyone who suffers from GID or is transgendered). My brother is happy dressing in women's clothing and walking around as a woman. He is more secure in himself as a woman.