If you're unhappy enough to cheat, you're unhappy enough to leave. I think people who are unfaithful are repulsive, actually physically disgusting. To betray the level of trust, respect and intimacy that spouses have for one another is a sign of pure selfishness. And if those things don't exist anymore due to the actions of the other spouse, as I said, the options are leave or rebuild, not engage in some sordid, grubby lie.
The level of arrogant, deliberate, manipulative duplicity you mention- pretending to be friends with the betrayed spouse- borders on the sociopathic.