I'm in a big pickle and need some advice

So we were getting desperate for a baby sitter. My best friend offered to watch the baby permanently when I went to work. So did family but none of them listen to my rules and routine that the baby follows. I trust her like a sister so I said yes but she would have to wait a few weeks because my dad is watching him and enjoying pawpaw time. However a few weeks ago she was talking about doing crack with a few of her friends and do things every now and then(not legal things)...I never got into her business but she was explaining a story. She also has them over every now and then. I know bad mommy I forgot all about it. I felt like a complete idiot knowing she did it and still does and I was going to let her watch my baby. I love her and do not judge her based on her decision but I can't let my son ever be around that environment. My dilemma is telling her she can't watch my baby. I don't want to hurt our friendship. I don't do those things and won't ever. Please kindly give me some advice on telling her she can't watch the baby. Also please don't judge me on me forgetting about such an important thing. I am being so picky now that I am back to work.