I think it would probably depend on the person. In general, they are probably unrelated matters. Each illegal drug is different from the next and alcohol is different in it's own right. So, in theory, it could be completely fine. One could be addicted to hard drugs and not happen to enjoy drinking. In that case, I'd say it wouldn't pose any problem. However, some former addicts might have a very addictive personality overall and be very driven to drink to extreme excess. Obviously, this would be bad. Also, drinking could be heavily included in their memories of using drugs. For some, triggering those memories might be triggering for them to go back to drugs. So, I'd say it's probably variable. Then again, I am not an addict, so I cannot really say.