Let me say up front that I have never had alcohol (nearly 40 now) nor do I anticipate that changing - a combination of not really being interested (I am the sort that prefers juice, water and tea over pretty much any other sorts of drinks), and having weak faith in this area (I can't quite get past how I raised to feel about alcohol). However, my study of Scripture leads me to the firm conclusion that its a matter of Christian liberty. I may not but I will defend other Christian's "right" to partake...if, as we should we all things, they do so with Christian principles of fellowship and love in mind.
Or he could have witnessed while he was drinking with them.
In one sense I perfectly agree with you. If your Christianity is tied to the belief that drinking is wrong, then certainly drinking will ruin your witness. If you believe that drinking is wrong then doing so could certainly ruin your witness.
However, if your well known belief is that God gave alcohol like he gave, food and sex - to be enjoyed in their proper context and with moderation, but joyfully as a gift from God. If that is known to be your belief then drinking alcohol in moderation and in keeping with love and concern for others, then if anything, it will *enhance* your witness. My sister brought up a good point. She said that unsaved people may see Christians drinking and wonder why they should become a Christian since they are doing exactly the same thing.
This is true, no doubt about it. However, it is certainly not an argument that alcohol is wrong. Why? Because the exact same things apply to things like sex and food. The problem is not with these things God has given for our enjoyment. You would agree that the fact that many people's lives have been destroyed by the abuse of sex is does not make sex bad or wrong for the Christian - certainly we would agree that such a conclusion would be absurd and unScriptural. The is true of alcohol. The real issue is not that it destroys lives (sex and food do as well) but that *when it is abused or used out of the proper context* it becomes dangerous (just as sex and food).
So, if sex and food can be rightly enjoyed by Christians in proper context and moderation, why not alcohol as well? Ecclesiastes seems to put it in the same general category as sex and food, so why should we deny this possibility?
Eccl 9:7 Go, eat your food with gladness, and drink your wine with a joyful heart, for it is now that God favors what you do. 8 Always be clothed in white, and always anoint your head with oil. 9 Enjoy life with your wife, whom you love, all the days of this meaningless life that God has given you under the sun— all your meaningless days. For this is your lot in life and in your toilsome labor under the sun.
Certainly Proverbs speaks very strongly against abuse of alcohol...but it gives the same sorts of warning about sex and food. So, avoid the strange woman, but enjoy pleasure with your wife; don't be gluttonous, but eat your food with gladness; don't be drunk, but drink your wine with a joyful heart.
I am of the same personal opinion - give me a nice cup of tea any day

However, this doesn't mean other's might have a differing and perfectly valid perspective about what's worth their while to drink.