A million factors to take into account, but usually just be there to pick up the pieces.

Bring it to the attention of the "wronged" party and the messenger is likely to be shot.

Bring it to the attention of the infidelitor and you generate all sorts of guilt based feelings, few of them good or constructive.

If you must interfere, do it through so many layers that neither can point at you as the interferee.

It may be with tacit approval, who knows?

__________________
Calm, quiet, smooth, devastating