...have their six.
***
What does that mean?
I think that it means to have their backs. Standing behind them.
Having someone’s six, or watching someone’s six, is an old military term that means “I’ve got your back,” or “I’m covering your rear.”
It comes from the hands on a clock, which resemble the points on a compass. In aviation “six” corresponds to the “six o’clock” position (on a compass that would be 180 degrees), which is directly behind the aircraft (and “six- o’clock high” means behind and above the aircraft; the movie “Twelve O’clock High” referred to enemy being directly in front and above the aircraft).