I think if you check this out (independently), you will find that she was cheating on you again, that the fight was just intentional "fight-picking" by her (I'm guessing she started it?) and she was using that as an excuse to exit the marriage and be with OM.
She does sound conflicted, and like she wants/needs your friendship. If I were in your shoes (and I was, three years ago this month!), I would work to verify the affair, and I would dial WAY back on the "friendship" thing with your wife.
In ANY event, "that argument sealed it" is bullchit.
