I read a bit closer. This is a wild guess, but...

...I'll assume you are in a typical, secular western euro country like germany. Mormons are so orthodox and religious that in that culture, she feels like an outsider (alien) because she takes it seriously in her heart/mind but knows others think its silly and backwards. At the same time, she bends the rules somewhat because its fun and that's what everyone else does, but feels guilty because she does think its true.

I imagine that's what she is saying/feeling. I think you misinterpreted it a bit about the alien thing -- that's not a literal mormon belief.

Super Brief Summary of Mormonism:

Take a methodist (generic protestant), and imagine that jesus came to america in 0 BC and wrote a book, the book of mormon. Church was "restored" by Joe Smith in USA in the 1830s, and quickly evolved from a nutty polygamist cult to a more or less mainstream christian church. 95%+ of the day to day life and beliefs are the same as any other christian church, except the temple (which even most mormons think is freaky, and chances aren't she hasn't been), they claim to have a living prophet (who doesn't actually do anything), and they are more centralized bureaucratically (lots of utah culture). But still its mostly generic christianty -- pray, read the scriptures, love thy fellow man, etc.

So if I am on point, the main conflict is she believes in her heart (not 100% in her actions), and knows others think religion is a relic of a superstitious age. From your perspective, probably just showing you respect her opinions, even if you don't agree, is enough to make her open up, and from there, you can handle it better than we could.