The fact that they left a person behind shows their level of experience.  The slowest person goes first, or if the group is breaking trail, second.  If they become exhausted and things become unsafe, everyone turns around.  I certainly don't have the experience that you do, but when I took AST-1 a few years ago there were a few simple rules:

1) Check the Avalanche report and know when not to go.  If unsure about anything, don't go.
2) Never leave anyone behind and never travel without a partner.
3) Beacon, shovel, probe.
4) Treat your beacon as a tool for rescuers to find your corpse (or at least your torso).  If you are involved in an avalanche, that is likely all that it will be good for.
5) Stay the !@$ away from cornices, avalanche paths, terrain traps, and be extra cautious around convex rolls.

My deepest sympathies to their loved ones.