Hey there,
I found this is a tough one. I've been finding it pretty ... um ... involved to figure out exactly how to handle drinking buddies. I decided ages ago that just writing people off because I used to primarily get drunk with them wasn't fair so I pretty much let things pan out 'organically.'
I think the big thing for me is priorities. How important is drinking/ drugging with this person? As I've found, if it's really important, the relationship is gonna drop off anyway because I'm not going to do that.
There are particular people in my life who do, indeed, seem to be just waiting for me to relapse. I know they'd actively encourage it if I did. I'm in the process of ditching these people, cos I think it's crap :=]
It is sad losing people but, ultimately, if they're going to bail because I'm sober then I didn't actually have a meaningful relationship with them in the first place. I was just kidding myself. Take care.