Grandaughter problems, any advice its getting really bad?!!!?

my grandaughter Dylan just turned 15 she was always a good girl but recently she started dating a boy called cory, who's daddy owns a ranch just down the rode, since then she has been found with condoms, she came into church drunk - on a Sunday morning! - she fell over while out with her friends and got bruises on her face, at school she told the teacher that her mamma had hit her ! Now she has asked to move in with me and I'm aware that it would kill her mamma if she left and also she is walking distance from cory but I don't know what to do! Should I let her move in? And what do you do about this behavior ? Could somebody please help me? I'm desperate!