Hi,
I hope it is going to be ok. When you calm down you should talk with him honestly how it was and what your fear is. If she is really his best friend, it would be wierd if you wouldn't know her. You have to tell him this. I don't know the background but he should also find himself safe, male friend who would talk with him about his own issues, that he isn't ready to tell you or are hard for him to swallow and he would need someone else to share this. I know it is very emotional for you but it is crucial to stay calm and loving. If he goes through hardships and comes back to drugs he wants to be away from, he really need help and someone to listen and be with him without adding the negative emotions.