Wanna_bump, I believe you that your heart is in the right place, but also don't think other people have necessarily missed the point.
It's a really individual thing how comfortable new mothers are with leaving their babies, and unless your friend seems worried about how little she goes out and how much time she spends with her baby, I don't know that it is your place to tell her that she 'should' do things differently. Personally, I don't think there's a big risk of creating separation anxiety in infants by spending 'too much' time with them, but one way or another, that's your friend's decision to make, and if she's happy, then it doesn't mean she's wrong or bad if she chooses not to 'take your points on board'.
With that said, maybe the most supportive thing to do is not to drop hints that she 'should' go out more/leave her baby with other people/have 'me' time, but just to talk to her directly as a friend. You could try saying that you notice she doesn't go out very much at all and that she seems anxious about leaving her baby with others and ask her if she feels that's a problem for her - if she feels pressure to be with her baby all the time, or that she can't trust other people, or if she feels she needs a break but can't take one for some reason - or whether she's just really loving being with her baby and has lost interest in doing some of the things she used to.
Then, as a friend, do her the respect of listening to and believing her answer, and go from there.