Thank you for all the great advice ! 
I looked at the post about books people are reading, and one that stood out was Beyond Addiction. So I bought it, and I’ve begun reading it. I’m not that far yet, but I feel it matches how I am and would want to handle the situation.
My friend seemed to get better for a couple days, and then headed right back down the path of relapse, and seems worse than before. He stayed at my place last night, but didn’t show up til after midnight after I’d already fallen asleep. He then said he had a stomach ache and ended up passing out in the bathroom. I heard him up at 4 am, so I went and checked on him, and he was stumbling around seemed very out of it. So I helped him lay back down. When I was taking him to work, he seemed loaded. Again, denied it, and I told him it would be better for him to take a day off than to lose his job. He said he had been to work loaded before and nothing happened. So I asked again, so are you loaded then. He still said no. I told him I didn’t believe him, and he just shrugged. I took him to work, I can’t stop him. And if he loses his job, that’s his consequences.
I’m so worried about it. I’m not angry still. Just frustrated and sad. I miss my friend and I hate watching him do this to himself. I’ll still be there for him, but I do need to take care of myself. I plan to still be in his life, but make sure I am doing what I need to do for myself, first and foremost.
I know I can’t talk to him when he’s like this. I am realizing that, and I think I read that as well. I am trying to get him to be honest with me, regardless of what it is. And hoping that will help him to be honest with himself. I’m kind of at a loss as far as what to do at this point because I can’t talk to him like this, and I don’t want to shut him out. I’d rather he be somewhere safe when he’s high, whether that be with me or his parents. Would just being there for him/with him even if he’s high help? ! 
) would help to eliminate other risk factors.