How can i be supportive of something that i don't support?

Long story short (or as short as i can possibly make it)-
My best friend is a young single mother of two (different fathers). We'll call her "K".  She likes a boy, we'll call him "W".
K and W met online about 8 years ago. They online dated on and off for about a year, getting together to actually "meet" only two or three times.  I was never a big fan of W to begin with, but he wasn't too bad at first. After about a year of online dating, K was going through a lot of personal issues and was in and out of psych hospitals a few times. W just couldnt handle everything going onwith K and broke up with her. After the breakup, W told K he actually had a "real girlfriend" the whole time anyway, and only talked to her when she wasn't around. K was devistated.

W continued to try to talk to K alot, telling her he loved her, etc. K had a baby with someone else but continued to talk to W regularly. She always still felt like she had feeling for him. They tried online dating several more times... but she always found out that he was pretty much always cheating on her. Each time this happend her world fell apart and i was always there to help pick up the  peices, over and over again. K hadf another baby with sojeone else.

W always talks down to K about her being a single mother, He also hacks into her online accounts like her e-mail and facebook... he writes mean/embarrasing things then changes her passwords so she cant get on to fix them. He always admits to her that he changes them and tells her what they are a few days later.

Personally I dont know why she still talks to him at all. In my opinion she should change her email, block his phone numbers and just forget him, but somehow she says she still has feelings for him. She says that she wantw to try giving him another chance and dating him again, but she  needs my support.  I want her to do whatever she thinks will make her happy;... she says hes changed, but i dont buy it. I dont want to tell her that i don't support her... but i just know this is going to end badly.

I guess i'm trying to figure out how to be a good/supportive friend, whithout really supporting the relationship. But like i sai we're best friends and i want her to be able to talk to me about whatevers going on in her life, and that's going to include him.  

I assume "K" is still receiving some type of out-patient psych care, given her history?  Is that the case?

Thanks ladies.

K texted me today saying "W is an asshole, i found out he has a gf" ... it just seems like this happens every year or so. She knows she shouldnt  be with him.. then some time goes by and he tried talking to her again and she falls for his sweet talking ways again and believes him when he says he's changed.

She def does need some continuing psych treatment, but i don't think thats going to happen.

I just have to keep being there for her i guess. Its a shame, she really is one of the best friends i've ever had... but she's socially awkward, and making friends is hard for her... she craves the attention of men and does anything to get it.... i'm at a loss of what to do for her.