It is still not right that he (the FO) didn't noticed until that late in the approach that the gear wasn't down. In my opinion, every modern aircraft out there is built and designed in such a way that you can fly it single-pilot if the aircraft is in a "normal state".
Why didn't the FO complete the approach by himself and later telling the captain in friendly words that he felt it was not proper what he had just done?
Why the FO wasn't sufficiently in the loop to tell or know the gear wasn't down until 500ft?
How hard is it to ignore a phone? How hard is it to put it away from yourself if it continues to bother you?