If you were unclear on the answer, this is because around 1000 years ago the German rabbi Gershom issued a ban on polygamy - probably to reduce the persecution of Jews by Christians, however most Jewish authorities rationalise it and insist it was for practical reasons such as the fact that two wives are more expensive than one. This was accepted as law by the Ashkenazi Jews in Germany and France, but not by the Sephardic and Yemenite Jews, who continued to practice polygamy in Spain and the Middle East, but the practice very gradually reduced in frequency over centuries as Gershom's ban became more widely accepted. Good summary from a Jewish source
here.
It is important to note that every sound Jewish source I have seen on this states clearly that Torah does not forbid polygamy, only that it appears to present monogamy as ideal and states that the Rabbis have required monogamy for practical reasons. For instance, the
Jewish Encyclopaedia. So if he thinks we should follow the Jewish interpretation of the law, that's awesome, just show him what they say it teaches!
