I agree with dale. you gotta warn her, if he cannot be convinced to do the right thing on his own. he doesn’t have to come out, but he should not marry her. he is intentionally ruining someone else’s life and he is doing it willfully. he has to be stopped.
in my opinion, i think you should try talking to him again and tell him how you would feel if someone did something like this to one of your sisters. maybe he’ll see the light? if he doesn’t, then you gotta to find a way to warn her.
i would hope and pray someone would have the courage to warn me!!!!!!!