Another thought: JWs believe that the only scriptural end to a marriage is if there has been infidelity. In my case my husband was an elder and had been abusive for years. When I left him the pressure to go back to him from all sides was pretty intense. I slept with another man in order to get that pressure off my back. In my confused and emotionally battered state it seemed the only escape.
I do believe that marriage is sacred, but I think that if you still believe, as the JWs do, that there is no good reason for needing to get out of a marriage and that 'families' should never break up for any reason... then you are still caught in the mind-trap.
In my opinion the reasons for the marriage breaking up are a private matter between the couple. Nobody else ever knows the truth about it. You should offer support to either party if asked for it. Both of them have suffered. However, if this person is still a Witness at heart you might want to limit your contact with them. After all, they could end up shunning you again after they go back. Offer your support, but wait and see where they are really at. You may have a chance to help them see that its really better on the outside. If they have cheated on their partner they are obviously wanting to find out about life on the other side anyway.