Friends ...

This is more of a relationship thing and it doesn't have to do with sex or anything like that but I need advice. one of my best friends has decided to do drugs like weed and before it was once a month but now it's almost every week day after day and my friends and I are getting worried and we want to talk to her but we don't know how to start or where to even begin ....please help we are graduating this year and its seems as though she is throwing her life away