Can diet pills lead to an eating disorder?

my ex girlfriend is a size 6-8 and she is now taking diet pills im quite concerned by the fact she is taking them, can they be pyschologically addictive and is there any chance of her getting an eating disorder.

I would say that it is likely that she already has some form of body dysmorphia if she feels the need to take them at a size 6-8, so yes she may go on to develop an eating disorder.
I think you should try to persuade her to talk to somebody about it.

Yes, tell her to stop
Is she obsessively exercising? and really obsessive about food? If so then it sounds like she may have a problem...Depending on her height, a size 6-8 could mean she is already under weight. I dunno what to suggest i'm afraid, i know that when i get into my head that i'm over weight no body can prove me other wise. Talk to her about why she's doing it.
I don't want to scare you but I know people who have taken them and they now have permanent damage (mainly in liver I think). I also hav e a friend & his sister died because of some diet pills. This was years ago (10) and they have evolved the pills since but she should go to the doctor & do some test (blood test) which will determine if her organism allows her to take diet pills & which ones to take (they are not all the same).

It's very nice of you to help her but I am afraid she needs professional help.
Not from the pills themselves but from the mentality behind them she might. The problem is as fantastic as it is that you care so much about her but the only thing you can do for her is to just be there for her and tell her that you appreciate her just as she is. Also try and get her to lose weight in a more healthy way then pills. Just watch her closely, she needs your support.