My friend is in a bad relationship...

You shold try to talk with your friend about this. Maybe she doesn´t realize it. She should definately talk with her GF about that too. If her girlfriend really likes her, then she shouldn´t treat her like that. If she doesn´t like\love (i don´t know if she is also in amor with your friend) your friend, then maybe you should show your friend that and try to convince her to leave her GF, even if it´s hard. Don´t just tell her to break up with her, but you´ve got to really show her the examples she´s been mistreated.
I really, really hope that you´ll be able to work things out and that you´ll get your friend back, without anyone getting hurt or something. Good luck!! :)

amor can be blinding and people will try and change so much about themselves to please those they love, they don't realise how drastically they're changing and how this can seem to other people.

You can try talking to your friend but in my honest opinion i don't think that would help, it will only push your friend further away. It would seem to her that you are not supporting her. You could voice your concerns, but ultimately i would visit your doctors surgery and ask for information on any charities or help-lines that deal with relationship abuse, domestic violence etc.

Talk to the professionals there and voice your concerns to them, they deal with things like this daily and can give you conselhos on how to proceed with your friend to get her to understand.

You could also try talking to her girlfriend, ask her why she would change your friend so much and stop her from seeing you and her other friends. Try comforting her with the thought that you wouldn't be taking her away from her, but just carrying on as usual.