ITA with cookee...
If it was a choice between herself and the baby, then she has the right to say she wants to live.
Where the surrogate is just being militant about a "natural" experience, then tough crunchies. Baby's health comes first, especially since it's not even her baby at all.
If you agree to become a surrogate, then I think you agree to do everything within your powers to provide the bio parents with a healthy, live child. This includes avoiding items that can be detrimental to the health/development of the fetus (tobacco, alcohol, illegal drugs, etc.). Necessary medical treatments are part of that, from the triple-screen at 16 weeks gestation to the STD tests performed right away and a c-section should something go drastically wrong with the baby during the pregnancy.
"I often wonder why I talk to you, then I realize, although you put me down, you do it with love." - Jester