Drinking Alcohol is forbidden in Islam whether its 'social occasional drinking' or habitual drinking.
a) They are not hurting anyone. may be and may be not. for instance, drunk driving: They may hurt others or hurt their own selves if apprehended.
b) Yes we cannot pass judgement on them but at least tell them [as you did] that it is wrong. After that it is a matter between them and Allah and may be between them and the state as well.