The reason voice training on its own is often not considered sufficient is exactly like what asi describes - too much effort, can't keep it sounding good all the time, etc. So if her low pitch keeps her from even reaching the female range at all, surgery of some kind is about the only option if improvement is needed (goal being, treat the psychological impact of gender dysphoria).
There are two surgeons in the U.S. who are considered pretty good at voice feminization. Dr. Haben was already mentioned, I had a procedure with him that went well. He made his mark by pioneering laser technology for treating throat cancer without radiation, and he offers a few different types of relatively minimally invasive voice surgery, so he might be able to come up with something for you.
The other is Dr. Thomas. His personalized procedure is to completely deconstruct and reconstruct the larynx, changing its size and dimensions, so if you feel a more drastic approach is warranted, this would be the man to consult. He also does less invasive glottoplasty, laser tuning, and even revisions of other surgeons' work. If I were in your position I'd definitely get in touch with him.
