I don't want to be a tattle-tail on this girl but how can I tell her mother?

My next door neighbor is home all day because of summer. I am home during the day 2 days out of the week and every day I'm at home during the day I see this boy go into the house after the girl's parents leave. Obviously it's a boyfriend and she is hiding him from her mother. The boy is a horrible influence on the girl and they are smoking (underage) in the backyard and probably having sex inside the house. I just want to make her mom aware of it. The girl used to be so nice to people but the other day the old lady across the street was sitting in her rocking chair and I guess was looking at the young girl and the girl said What are you looking at Old HAG! No doubt it's this boys influence on her.