My bf just got arrested for soliciting for hire, do I leave him or not?

We've been in a relationship for over 6 yrs and everything that can go wrong has went wrong in this last year. He just told me he does meth. He did meth all day yesterday and all night last night. He calls me at 9 am this morning telling me he got arrested for drug paraphernalia and soliciting for hire. His story is... he was in his truck with his freind (Will) and Will's cousin, some girl they went and picked up. Will went inside of someone's house, which left him and the girl in the truck, the police pull up and question both of them. The police say she is known for soliciting so they arrest him and let her go. Do I beleive his story? Why didn't they arrest her too?