In the first pregnancy it was unlikely that her being Rh negative was what caused the problem because the mother and baby's blood usually don't mix until delivery, but she should have been given the shot after the first miscarriage, definitely.
All of what you're saying seems really inconsistent to me. The doctor said not to worry about it, but confirmed that it was the reason for miscarriage? How did she know that, considering it's pretty unlikely? It seems like if what you're telling me is true, this doctor would have been sued for malpractice by now.