I don't think that Arenas' career will survive, even with the most union-friendly arbitration setting.
Washington DC’s gun ownership/carrying laws are unConstitutional. If he is charged with “gun possession” I think he should fight it all the way to the U.S. Supreme Court.
However, if Mr. Arenas and/or his team-mate did BRANDISH gun(s) at the other or at one-anotehr, then appropriate criminal charges should be brought.
As for the league/team rules/punishment, IF there is a “no-gun” clause in his team/NBA contract, or if the league or the team have previously established “no-gun” rules for the locker-room(s), then he should just face his team/NBA punishment like a man.