What type of interaction? how deep should it go? How long should it last? It is for you to decide.
I don’t think Crystal is ready to get married! She still doesn’t know what she wants if she is asking these questions. Just like you mentioned “it is for you to decide” If I could advice her, she should wait with her wedding, otherwise these type of marriages end in disaster. She may love her boyfriend now, but would you do something that completely changes your lifestyle and still have regrets, second thoughts? Im not an advisor, but that’s what I feel. If they let be each other...
Unless she and her spouse choose a polyamory (type of relationship)...
That is only temporarily solution. Long term it still ends in disaster. I can almost guarantee.
They both need to be in a certain type of consciousness, indeed.