CWB

it sure seems like there is a lot of "stuff" going on with her . you did not mention how old she is (or i missed it) .
her friend indicates that things are "normal" with her ... at least as far as she knows or is willing/able to tell about . to me it is rather odd that her "best friend" has absolutely no clue as to any "inside information" .
in reality there could be some problems with her home life ... however , she has chosen to not share/trust or make you aware/part of those things ... so be it , that is her right . the key is that it is HER choice/action and reasoning ... she has chosen to not want/let you be a part/knowing of those .

you have done your part by trying to communicate with her ... the slate is almost clean . personally speaking , i would suggest (based on my past) doing the next correct indicated thing : write her a very calm and rational letter ... no fault finding or finger pointing ... state that you have tried to understand her and accept her as she is ... that you cannot and will not live your life based on mystery and drama . let your words be kind (you wouldn't kick a sick friend) ... this will do more to help her down the road and you will not regret them should it turn out that there is some heavy stuff going on .

move forward .