My question is if you've been friends for such a long time, and you know her past habits - have they changed any since she became a mother?
I have a feeling that you going 'up to the plate' in this 'game' is just what she wants. Someone else to take responsibility that she is not ready and/or willing to take yet. You need to talk to her and see if she intends to be a good mother or continue her 'party life' for as long as possible.
How about suggesting that she only goes out once a week and see if she can take up her own responsibilities when you go out once a week on another day - so that she will have to be there for her child.
I'm a mother and grandmother, and when one is a mother - being the party animal will have to go to the back burner... PERIOD. I don't believe in anything else because children are more important - whether they were planned or not - they are there - and it's time to regroup and take over the responsibilities or give the child up to someone who will care for it more.
In my opinion - she can't have both.