I don't know how to answer this right now. I would have to go with my gut. If they said they were sorry, but still running around like they were before it happend and getting involved in the same situations that lead up to it.... Bye bye....
If they were willing to go to couple's counseling and took ownership of what they did and the effect their action had on the trust we used to enjoy, I think I would be willing to give it a go.
The key is, if it happened because they would rather be out of the shackles of a marriage or relationship, then it most likely IS going to happen again and they might as well save both parties the heartache by just ending it. If it really was a fluke or a combination of amazing coincidences, then I think there would be enough of a foundation left to attempt a recovery.