There's really no reason for you to discuss this with her at all
I agree with Scully. Your relationship with God is your business. It's private and personal. Now if she asks you, then answer and be proud for your choice, but you are doing what you have to do, just as she is.
My best friend is still in, and an elder still giving talks at district conventions. I'd do anything for him, as he would for me, but we've reached an understanding. Sort of our version of "don't ask, don't tell". I don't want to put him in a position where he is forced to make a choice, and I think he feels the same. But if he ever wanted to know, I'll tell him. I don't hide anything I do, and neither should you. But this is her day and it's going to be stressful enough. Be there to support her and leave this subject for another day.