My advice, for what it's worth:
Don't tell the bf
Let your friend know that you disapprove of her actions and why, - but don't expect her to change as a result. She is still the same person that you became friends with and that should be valued, even though you have differing opinions on this one thing. And offer your support and sympathy when it all goes horribly wrong, which it is likely to do! People have to learn by their own mistakes.
------------------
Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas