Eagler: doesn't her reported past actions indicate the otherwise?
I have no info on that, so I was speculating on very general lines.
or is the world to the point now that she can be a mistress AND a "decent person" too
That depends in what kind of decency we require from her. A person can be sexually promiscuous but would not sell out a friend.
In that case only his/her spouse should have reasons to worry. The label is unimportant. If you do not like to call such person "decent", I would not insist. The label "reliable" or would fit or such.
miko