Religion is not just a 1 hour devotion on Sunday morning where everybody can see you kind of a thing. You don't need the Church to follow the Bible but joining the Church helps you to grow in your bond with Christ and deepen your understanding of Biblical teaching. It creates community.
We all sin and feel unworthy of redemption at some point but we are saved through God's grace (receiving a saving we "don't" deserve because we accept Jesus as our Saviour).
If a Menonite feels that drinking a beer isn't so bad, then maybe he needs to consider another Christian faith that would permit him to do so. I don't see a prohibition on alcohol in the bible (in what I take to be an inappropriate request, Jesus turned water into wine). Drink beer, drink wine, drink liquor if you want; I do. But be aware of how and why you are drinking. If it is an escape and you seem to be drinking to escape more and more then maybe you need to reconsider drinking for awhile.
In the example that you cited, it was unlikely to cause any serious problem; but it violated his faith.
There is even a provision in the Amish tradition that has been making the news in the last couple of years. The teens and young adults are given the opportunity to act out and wander before returning to the fold. This includes using modern devices and drinking and has been reported to include even illicit drug use and sex. These actions would appear to be outside of the Amish tradition but accepted with some framing (it will be given up as they return to the "tradition").
I don't ned to learn about the Mennonite faith. Hubby has several Menonite preachers and a deacon in his very near family. What I've learned from Hubby's relatives hasn't warmed me toward their style of worship.
You have the correct keyword in "tradition". Interpretations become traditions. Traditions become sub cultures. Some of us are uncomfortable within those subcultures.
Amish...don't even go there.