Righto, I'm jumping in!
Believe it or not, there will be the occasional young teenage couple that are somewhat mature enough to handle something like marriage. It doesn't happen very often, but it happens. It is a rare thing, though.
Given that, I don't know about the GUY because he's been verbally abusing the GIRL, and that's not good in any relationship.
That said, people do change.
If the GUY is indeed doing this as a way to continue to manipulate her, or if you at least think he is, you should be a good friend and talk to the GIRL about it. Don't try to impose anything on her though. Just be very VERY polite and calmly explain your worries to her, all the while letting her know you're merely trying to be a good friend who's looking out for her safety. She should know you're just concerned about her, and you are afraid something will happen to her, you can't handle the thought of one of your closest friends being in a situation where she is regretting everything. I cannot stress this enough!
I'm thinking the GIRL may be a little too naive, and naivete is incredibly dangerous. You should probably tell her to secretly be on her guard and not let the GUY know you were talking to her. He might get a little huffy and start hurling abuse at you as well for "meddling".
I think it's a little foolish to just sit back and just trust in God to "fix" this, because after all, there's every chance He might decide to let it happen. Other people might tell you meddling in her affairs is a very bad idea, but I'm telling you now, if you trust in God, He might plan on USING YOU to help get it sorted. If you go talk to the GIRL like I've said, this may be God's way of trying to help your friend.
I do think though that ultimately, she has a choice in this matter and you are merely there to try and influence her. It's a battle between you and the boy, with her life as the "prize".
It's likely to be her choice in this matter much in the same way we decide whether to follow God or not.
I really hope this helps. O_O
Ark