Don't let her try to bring in the missionaries to explain; remind her that she is an RM and knows all they do and probably much more. I also think that if marriage outside the covenant is a sin, it is not so grave as to be unforgivable.
This can keep the relationship from getting too exclusive, and spending time with more people can give you a wider variety of things to do. I look forward to reading more.
Please see above link for full rules.