There is really only one option, isn't there?
Why are the girls family trying to 'cover it up'? What benefit can it have for them? Their daughter is the one whjo has been wringed here, not your friend. I see something a bit more going on here. How come the brothers are not beating him to a pulp? (not that i condone violence). I do not understand the families attitude at all
However, saying that, if he is working towards youth work where he will meet vulnerbale kids, then there is some responsibilty here for you to report him somewhere. Where, I do not know, but such a serious allegation would be acted upon
ATEOTD, you know what you should and must do
GL