I think you should ask your friend why she started drinking again and if there was a reason she hasn't told you. I don't think, however, that you should be this from the standpoint or view that she is accountable yo you or that she should have been. I'm merely suggesting that you express the curiosity of it all to her just as you are framing it here. It does seem, after all, that you really are just curious. Being curious without bring judgmental is totally fine if done correctly, I believe.