notb, the very fact that you have asked here and not just ignored and dismissed what is going on, shows that you know there is cause for concern here.
You are an adult and can make your own choices but this very young child cannot.
I feel for you very much, it is very hard to stand up and be counted with issues like this but you now that you know what is happening, you have been put in a position where to do nothing (though initially easier for you) would be the wrong decision to make and I feel that you would regert not getting help for this girl and ger child.
These days, everywhere you turn, people and agencies say things like nothing can be done if you dont leave your details.
This is not so with child issues.
The mum may well be very scared that she will lose her child if anyone 'official' finds out.
This would be a last resort.
There are agencies that can help her, or at least help her to cope with her child while she fights her own battle with depression.
People have mentioned how self harm does not have to mean that she is a bad mother and I agree entirely but in this case, you have mentioned situations where the child has been at risk.
Please notb, do tell someone so that both the girl and her child can be helped to cope better.
Any information that shows that a child might be at serious risk must be acted on by agencies, it cant be ignored but you do not have to give details.
If you feel able to pass on this information then other people can step in and help.
If this is a friend or family, it can be very difficult to 'intervene' or to be seen to be pointing the finger or prying, etc.
The people who can help do not have to shoulder the responsibilty of perhaps being seen to betray a friend, as people sometimes wrongly but understandibly feel.
People who care and can provide help shoulder the burden of prying, delving, asking awkward personal questions etc because they are looking to find out if there is a problem.... they are not looking to win friends and if people get offended by welfare checks being made then they deal with that.
It is better being safe than sorry.
long winded reply notb but I dont apologise.
Its hard for you and if it was just the girl, then my personal advice would be different.
It is a totally different different ball game with the child.
I hate the term 'just crying out for help' but your friend sure sounds like she needs support and help and someone believing in her.
Be reassured that there are people who want to help.
Im not saying it is easy notb... but if you look at it as helping and not making trouble then maybe it will help?
For what it is worth (and only because of the child) I think you do something.
(I am assuming that you are not able to discuss it directly with the girl and encourage her to seek help before you do)