J.T.
You need to tell her how you feel. If she is truly your friend she will accept that her actions/attitude have been hurtful and will make an effort to change. If she doesn't then you need to move on and find a true friend for whom the relationship is actually beneficial to both parties. It sounds to me as though you have taken it upon yourself to be responsible for your friend's child...admirable, but I wonder if it has more to do with your desire to spend more time with your own child than with helping her out? It just seems like there's not much reason for you to be in this friendship and maybe you haven't dealt with this sooner because there is something more to it. In any event, you can't ensure that you won't look like the bad guy. The reality is that she will probably be upset/irritated regardless of what you say, what you need to remember is that your feelings don't make you bad, regardless of how she perceives you. All you can do is figure out what your boundaries are, tell her why you're making them and give her the opportunity to respond accordingly, whether favorably or not. And don't forget to be prepared to stick to your guns and cut your losses if need be.