I've been in a situation like this before, not necessarily involving a baby, but a controlling girlfriend who manipulated her boyfriend into depression by forcing him to stay with her.
I made a list of everything that his girlfriend did that showed that her love for him was shallow and just manipulative. I wrote every single event that happened that she started that made him feel like s**t. Then I pointed out every time he threaten to leave, she would pull him back in with crying and false promises.
Seriously, I wrote down everything and confronting him, begging him to see behind all this at the person he was really with. Look past all the lies, all the negativity, and all the empty love.
Never once did I say dump her or anything, I just wanted him to see from the outside in.
Within a week he broke up with her, and he was happy again.
I don't know if it will work in this case, but maybe your friend needs to see from the outside in.