I have never met a drug addict who can successfully drink. Ever. I have only ever seen two outcomes: 1. it's a gateway that leads them back to drugs; or 2. they experience cross-addiction, and become addicted to alcohol as well.

This is just what I have noticed. The thing about addiction is that everyone feels differently about it. Some people are of the 'once an addict, always an addict' mentality, some people believe there are different degrees of addiction, some people believe it is okay to quit drugs and still drink (or quit drinking and still do drugs). At the end of the day, it comes down to what kind of effect it has on one's life and the lives of those around them, and what they are prepared to sacrifice (or not).