Wow. We weren't discussing her mom until just now when she answered a question. There was no need for the "slut" comment. That's probably why she didn't mention that bit at the outset because of people like you.
That unpleasantness aside surfhb is correct in that technically your mom was cheating too because she knowingly enabled someone else to cheat. That is just as bad as being the cheating party who is in the relationship. Not knowing you are sleeping with a married person is one thing, but knowing is quite another.