I never lied, until I drank alcohol, for years; then I became expert at lying to myself and others.
But, the locking his child in a van to drink, that's a separate issue from lying. I did many abhorrent things under the influence of alcohol, but none involved the safety of, or cruelty to children, adults or animals. He may have then driven over the limit, thus placing his child (and others) at further risk.
I too agree that you should inform the child's mother of this event - if he locks a child in a van to drink.....what will he do next?