I am third generation JW. My mother married when she was 15. My sister married when she was 18. My other sister married when she was 17. They will make sure they are married. Probably this summer. I'm so sorry. Always keep the door to communication open. She'll need someone to turn to once she realizes the truth.
Also, this exact same situation happened with a family where I grew up. They moved the girl in with them. Helped her finish high school, got her baptized, and married to their son before the end of the summer. The local JW busy-body sisters even got involved and helped her dye her white wedding dress in tea because "white is supposed to symbolize purity" and she was not because she wasn't a virgin anymore. It sounds so cultish as I type this out.