I think that the death of a young mother, whoever she is and for whatever reason, is always a sad situation. The fact that a death may be preventable because it was caused by mis-use of drugs adds another layer of sadness.
Addictive behaviour comes in many forms: alcohol, drugs, sex, food, gambling, cleaning etc. Most research I have ever read about has indicated that many people with 'addictive' genes do not develop major addictions but it is their environmental experiences that, when combined with the genetic propensity, can cause major addiction. Of course a developing foetus will be affected by drugs in the womb and that damages the development of the brain but presumably it could be possible for a baby to be born, addicted to drugs, but without the addictive gene?
I am undecided about how far genetic research should go. Are we looking for a future where we are all 'standard' humans? Where will we stop? What does it say about how we value those who are born with particular conditions/ disabilities? I agree that finding cures is important, I can see why nobody wants a child to be born with a disease but are we clear where we would draw the line?