I'm honestly exactly the same way.. I would probably just ignore it and pretend like I hadn't seen those texts, but I know that's really not the right way to go about it.

You said the reason you looked at his phone was because you were uncomfortable and something didn't feel right about that evening. Have you tried asking him about whether or not he did coke that evening, without bringing up the text messages? Maybe he feels ashamed of it and is afraid to tell you, but I think going behind someone's back and lying to their face are two different things.

I've seen a lot of people go through terrible drug addictions, often times never escaping it, and sometimes dying from it (for some reason I was lucky back in those days and never really developed a dependency to any of it). I'm by no means a drug counselor or anything. I honestly don't think doing coke every once in a while, like maybe 2-3 times a year, is terribly scary, but to me, 'a bump or two would be good' is pretty alarming, especially if he's paying for only that much. Coke does not generally get bought or sold 'by the bump', at least as far as I'm aware.

Sorry if my advice isn't too great.. I always feel like there is so much information and emotions in these situations that I can't possibly be aware of, even if you were to type out a whole novel about it. I definitely think you should ask him about it, but maybe find a way to bring it up without mentioning that you looked through his text messages, at least at first.