it's not right, but I mean, the thing is, most people do go to hell, thats already said so in the bible by mary, but beside that point.
At this point you have blame both your friend and the girl, I mean, her parents must be doing something wrong that their first daughter would go into drugs and their second would actaully date someone 12 yrs older than her.
You just have to tell him it's wrong, beyond that, theres nothing much left, life goes son like that.