Boyfriend broke his promise

Basically, the guy I'm in love with went out with his friends and did some less than legal drugs. I just don't know how to react. I am not a partier, but enjoy going out with him and his friends, because it let's me learn about him and makes us closer. Last night he went out without me, went bar hopping and despite saying he never would, did a line. That is not the kind of person I want to be with, but I love him, and know that he hates himself for doing it. He told me he feels as though he broke my trust and kind of jumped off the bridge just because everybody else did. I just responded with "happy 1 year", as it's technically our one year as of this morning, and left him on delivered. What should I do? I don't know who to talk to about this, and I don't want him going down the wrong path