R.S.
I have a little different spin on this. I think you need closure. The thought of leaving it hanging the way it is bothers you so here is my opinion.
She was a good friend of yours before the kid came along so perhaps you might just give her a call (or email her again if you don't get to talk to her person-to-person) to say that you appreciate her past friendship but your needs have changed since your lifestyle has changed and a friend that takes an active interest in your mother role and your kid is important to you now and you understand that people grow apart. Wish her love and blessings and ask her to keep in touch.
I have had a similar situation in my life (I think we all have) and this ended up being a good way to transition our relationship to where it needed to be and we both felt understood and respected.
Good luck!