Over reactions abound in some of the responses so far - they are understandable but unhelpful.
Truly, young people are at the mercy of their feelings for their boy- or girlfriends. It ain't got anything to do with abuse - it's a developmental fact. I've seen young guys become simpering wrecks due to the love of a lovely girl and vice versa. Add in the boy or grilfriend's religious background and you've got a recipe for emotion-based conversion - regardless of the religion. What are all those love songs about "I'd do anything to prove my love to you" - yes, even join your religion.
Be warned that anything you do to open this young girl's eyes will be construed by her as you trying to come betwen her and her "true"love.
On the plus side, these young love relationships come and go like the summer rain. This month it's infatuation with a boy who belongs to the JWs, next month or later it'll be infatuation with a boy whose family are (shock! horror!) Moslems.
Panic doesn't help - in fact it drives the person further into their love relationship.
Never forget: Even if her love for this guy endures and she converts, the exit door at the back of the kingdom hall is getting bigger as time passes.
In the final analysis, though ,she's 16 - and in most jurisdictions, she's old enough to make her own decisions - whether we agree or not.