If he is driving and plying the road for business, then it is
absolutely wrong. The limit for off duty only, a pint of
beer, anything above that, is dangerous.
I understand your concern, a wise man will never argue with a
drinker, cause they talk no sense. It is sad that he will get
revoked, and if got young childrens, his family is going to suffer,
but then, everyman must answer for his own deed.
For me, I would not say yes or no, it depend also on your
friendship with him, how much you know him, what is the arguement
about, is there a way to teach him instead of calling for
police? These are all the question you have to answer
yourself. At the end of the day, he may hv to go home and tell his
kids and wife that he lost his taxi licence and job and it could
be, like you said, a blessing in disguise that he is no more
driving and the public is safe.