Sounds like your biggest concern is her appreciation for the risk involved, and consequences of not giving that risk the acknowledgment it warrants. If you're convinced that she is going to ride come hell or high water, you're better off supporting her and making her feel like you respect her decision to do so. Taking her out into the dirt is a great idea. Not just for the experience, but for the fun-factor. Sell her on that.
Suggest meeting this new boyfriend, go for a ride together and get a feel for the kind of rider he is. I'm guessing he'll have a stronger influence on her riding choices, as you've noted, so he's your best bet for getting through to her. Disagreeing with him will come across more positively than disagreeing with her, since you won't be out-right bashing heads with her. Or the inverse, he's a level-headed rider who could steer her on the path of rider-for-life, and you may have less to worry about than you think.
I've been in the same boat with some extended family and very close friends. Trying to convince someone they shouldn't do something if they don't do it your way is only going to push them to doing it any other way.
) I am planning on taking her to Horseshoe this season for an off road school...they have all the gear...different size bikes and experienced instructors.
