Facebook Help

Hello,
Recently i have noticed that my best mate has had some pretty uncomfortable conversations/movements with my gf . I told him to stop but he continued, since he's my best mate i don't want to accuse him of anything. So i want to investigate a bit. So i taught that my best bet will be to get a way on to this Facebook account. Now i don't know any way to get in to his facebook so i taught that i maybe can find help here.

The best way is to keylog / rat him, you can also see if the email he used for Facebook is real or not, if it's not, just create that email and use it to reset the pass back to it, or just confront your friend