I certainly sympathize with your heartbreak, but you're basically asking me to help you close the barn door after the horse has escaped. By allowing your minor daughter not only to date a Mormon -- a member of a religion that is very anti-Catholic and highly evangelistic -- but to attend his church with him, in an effort to show how "open-minded" you were, by your actions you basically gave her permission to convert. To try and stop her now is going to appear to her to be hypocritical and, given that she's nearly 18, may well prove fruitless. Please understand that I am not telling you this merely to pour salt on your wound, but in the hopes that you (if you have younger children) and other readers will learn from this mistake that it is extremely imprudent for committed Catholic parents to allow their minor children to date non-Catholics (much less to attend non-Catholic churches with their dates).
About all you can do now is to apologize to your daughter for failing her (which you did and by the admission of which may mitigate charges of hypocrisy) and enroll your whole family (including her while she's still under your authority) in a family home study of your Catholic religion. (Meaning that you are going to have to lead your family in a study of your Catholic faith.
Here's one possibility for a curriculum.) You may also ask her to read and write book reports on the books linked below. As for her boyfriend, I doubt it would do any good (and it may well harm your efforts) to forbid them from dating now. But you can insist that she stop attending Mormon church and Mormon church functions while she is still living at home with you.
Recommended reading:
When Mormons Call by Isaiah Bennett
Inside Mormonism by Isaiah Bennett
What do I do if my teenager wants to convert to Mormonism?
Hybrid Mode
