Everyone's a rabbi

My son described the following from his bus ride to Yeshiva:

Bus driver and passenger are discussing water filters and bottled water.  Passenger says that she uses bottled water, as she cannot use the Tami 4 system on Shabbat.  She explains why she thinks there is a problem.  Bus driver explains how to get around it.  Woman and bus driver engage in long argument over whether his solution is halachically acceptable.

Woman was not specifically dressed "frum".

Bus driver was Arab.

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