Self-mutilation, sadness

hello, i am writing not for my self, but for someone who means a lot to me that i would like to help.
she's someone who has been diagnosed with borderline personality disorder before, and she used to cut herself, and has very bad scars from her past experiences, which was about a year or two ago.
she also used to think about killing herself, and has overdosed on drugs and been hospitalized before.

recently she just began to cut herself again..
she says she is constantly sad, although during the day things happen and she gets happy for awhile, she just returns to being sad at the end of the day. she doesn't know why she is sad, she just is..
and she also tends to think about suicide for no particular reason..

i thought that she was getting happier, which she says she is, but she says that the happier she gets, the sadder she gets..

i would really really like to help her, and i have talked to her, and i thought things were alright..
i'm not sure how to help her anymore, and i don't know if by helping make her happy, i am in fact making things worse...

please, any help or advice at all would be very very greatly appreciated...
thanks loads

Pl get professional help for her.