If it is not a frequent activity and the children understand the possible hazards of drinking, then it should be fine.
Smoking however, is another issue. Second-hand smoke (friends smoking) is detrimental to children and should be avoided at all costs.
There's a higher risk of your children contracting asthma.
http://sg.theasianparent.com/how-secondhand-smoking-affects-children-negative-effects-of-smoking/
http://www.livestrong.com/article/73239-emotional-effects-children-seeing-parents/