Dad has been drinking whiskey 200 ml every day for the past 20 yrs, blood work last week was perfectly fine, how is this possible?

He's lucky? One study showed that alcoholics who drank coffee on a regular basis seem to have less liver damage. He may have avoided other meds such as tylenol (acetaminophen) with the alcohol and some is just genetic. Other people who drank the same could have liver problems. Some times I am amazed as well with some of my alcoholic patients, however while the liver looks good, not all is well in their lives.
Very very common. There is a common misconception, even among doctors, that everyone who drinks regularly damages their liver. There are genetic predispositions to liver damage with drinking, and ultimately only about 30-35% of heavy drinkers get liver damage. Also depends on what blood work they did; they may not have done the most sensitive tests for alcohol use - ggt, cdt are the most sensitive.