Give her your opinion, I'd say in an email.
Things I'd say ...
Mental health problems and addictive behaviour can be genetic. If she's really just looking for a sperm donor, bear in mind - this one's faulty.
Addictions can get very expensive, salary, family savings and pension getting pissed down the drain.
The only relationship they have is with their addiction. She and baby will always be at the bottom of his priorities.
Violence can be self harm - and include harm to others too. Addicts and mental health sufferers may not control violent tendencies.
He can't cope with life now so how will he cope with a screaming helpless baby- and what if she gets post natal depression? How will he cope with that plus the sleepless nights?
This relentless pressure from her can't be good for his mental health now. What's it going to be like through a pregnancy?
Does she honestly think he'll be inclined to or even able to step up and support her emotionally?
And how will she cope with a baby to care for when she gets the call that he's gone off a cliff?
I'm actually starting to feel sorry for him.
But more importantly for the baby she's considering bringing into contact with this agent of chaos.
Then there's the social services - will they even let her keep the baby in these circumstances?
After you've written it all out, send it - then stay out of it. You may well lose her as a friend as a result - which may be the best thing for you.