Originally Posted by Dagferi
Ummm.... Polyamory is based upon ETHICAL non monogamy. Not banging your friends boyfriend then saying hey why don't we have a threesome or triad
Strictly speaking, ETHICAL polyamory is based on ethical non monogamy. Polyamory is just based on romantically loving multiple people. While I think this is an instance where most can agree that this is the "wrong way" to do polyamory, it still is "a way." Just like two people can get married and be cruel to each other and have an absolutely terrible marriage, but that does not negate its existence as "a marriage."
As for the OP, there will be two completely different issues to confront. Polyamory will be one, but the cheating will be another one entirely. They need to be dealt with as two completely different issues, not just two sides of the same issue.
Cheating is 100% about lying. Finding out that your partner has been lying to you for 18 months, be it about another relationship, a drug addiction, or having lost your life savings in the stock market, is a devastating blow. The lying itself is going to be the major problem there. That he was lying about being with another woman is just the object of the lie. What hurts people most when they're cheated on is that they were lied to.
They'll need a lot of time as a couple to deal with the lies and trust issues surrounding the cheating. That will have to happen before there's even a chance she'll be willing to consider polyamory. You need to be prepared that you might have to step away for months, maybe over a year, before they'll be recovered enough to move forward.