I think the problem is obvious... nobody who really loves you is going to wave crack under your nose and ask you to take a hit. Let me guess--drugs had something to do with why you broke up in the first place? I think you need to keep more than arm's length away. Preferably a different continent.
I can tell you still have feelings for him, but in my experience that never works with addicts. Two addicts tend to be as strong as the weakest link, which usually isn't very strong at all.
Anyhow, as long as it was an isolated incident it's no big deal. When you fall, you have to get back on that horse. (Wait a minute--bad word choice.)