I was raised in a very conservative Christian family myself so I have an inkling as to what she is going through as well. Best defense for Christians is a good offense:
I've posted this before (heck, I may have posted this earlier in this thread) but for other Christians having issues with swinging and religion, first remember that the Old Testament had almost every marriage being between multiple people and it wasn't 'condemned', it wasn't even said to be wrong. It was almost needed to provide the labor for farming or herding. It just kind of faded away as more people moved into cities instead of needing the help being nomads/farmers/herders. But just because having multiple wives came to an end, it didn't mean that monogamy was the only accepted 'norm'. In Roman times, just because you were married, didn't mean you never had sex with anyone else. People 'think' that the Bible condemns most sexual activity, but in actuality it doesn't, and the Romans had plenty: bath houses, legal prostitution, orgies, gladiators (not just for fighting in the arena), homosexuality, sex with slaves, and more were all considered legal, allowable, and not a violation of the marriage.
Next, read this:
Monogamy Isn't Biblical, It's Roman
Finally, remember search (hidden in upper right corner) is your friend. There are a bunch of threads dealing with Christianity and swinging.
Here's a good book on the topic:
"Unprotected Texts: The Bible's Surprising Contradictions about Sex and Desire" by Jennifer Wrigtht Knust
This ought to at least give you some ammo to start the conversation rolling. Let us know how things progress!