Have you talked with her about how Mormonism is not God's way, and have you shown this from scripture?
If she has been shown scripture for how Mormonism is wrong, why has she made this choice?
If she knows about scripture challenging her decision, you can advise her that if you go it will be in order to reach her and others who are into the Mormon thing. Then you can go and reach people for Jesus.
Or, say, No, because she did not talk with you about what she was getting ready to do, in case she kept this from you but now all of a sudden expects you to accept this without first communicating about this and evaluating this with you.
If you are not equipped in God's word for this, you might say you need more clarification about Mormonism before you can make a choice about sharing in their activities.
I personally have taken a quick peek at the Book of Mormon and it was nothing in comparison with God's word; so I just stopped, right there. Plus, if Mormon is really Christian, why do they use the word "Mormon"?
So, I might visit Mormon stuff, but in order to investigate what someone is into, and compare this with what is in God's word.
I have been told that Mormonism is a cult, and I know cult people can hide what they are really about. But, there can be untrue rumors going around about even cults, for all I know. So . . . one thing I do with any group's member is make sure the person speaks for one's own self, about what he or she understands a group teaches and does. Because that is the person I am talking to. And compare this with who Jesus is and what God's word says about salvation through Jesus and how Jesus is the way to God, and how a person needs to start new by trusting in Jesus.
"What is your actual experience of God and His word and salvation?"
"How have you become with God because of trusting in Jesus, if you have?"
And God does use prayer and your example . . . wherever He has you go. If He has you go, do not try to rehearse: He is able to do better and more than we can plan and hope now.