First thing to do is get her to a doctor as soon as you can and discuss your options with him. Not only does heroin addiction greatly increase the chance of miscarriage, but trying to detox also puts the pregnancy at considerable risk. Even at a very early stage, the embryo is dependent on opiates (because it is receiving them through the umbilical cord), and trying to detox cold-turkey has the potential to cause a terminal detox to the embryo. Depending on how far along she is, there's even a chance that the doctor may tell her not to quit until after the birth. In most cases, however, they'll recommend a program that will help her ween off in a way that won't be harmful to the pregnancy.
So sorry you have to deal with this. Good luck.