Oh LTG how I do sympathise with you.
My D had a difficult first labour because she has a slight deviation in her coccyx, and the babe got so far and could not get through - she was in hospital at the time and they pulled him out, managing to get him round the obstruction.
Pregnant again she booked a private midwife and organised a home water birth. It seemed the height of folly to me, as we knew that the babe was likely to get stuck, but she was determined and the midwife seemed unperturbed. In the end she laboured (in agony because babe was stuck) and then was ambulanced in to hospital for a forceps delivery. Lots of unnecessary suffering.
I was within a hair's breadth of telling the midwife that I thought she was making the wrong decision, but held back. All's well that ends well as babe is fine, but my D has serious piles which cannot be unrelated to the fruitless pushing.
On a more positive note, another D had a caesarian with the first and went on to have two normal vaginal deliveries - but both in hospital.
I do not know where this rejection of science comes from - people have worked long and hard over centuries to make labour safe and we are so privileged to be in this position. All this airy-fairy stuff about home deliveries is absolutely fine when it is a low-risk pregnancy, but is nonsense when there are known problems.
I cannot tell you how I feel for you - I have been in the same position and bit my tongue. Can you marshal some statistics from the net that might sway her mind? Or else, go to your own GP and express your worries and ask if they are groundless so that you have some concrete information to either settle your mind or make you decide to tackle it with her? The idea of going behind D's back is so hard - you jeopardise your future relationship, and I was in exactly the same boat, so my heart goes out to you.
One of the problems is that we, and our children, are so insulated from the realities of obstructed labour because we have the benefit of modern science - if we lived in parts of the third world, we would know only too well what the problems are and would grab the help with both hands.
I can only wish you heaps of luck and send my good wishes.