Sorry to hear this Mummy, I can see how low you feel re your daughters friendship issues but our DCs are stronger than you think you know so try and not get too down as your DD will pick up on this and it won't help her.
Firstly this sounds like a bit of a bullying issue, do you know of any reason why the original friend would behave like this? What did you know of her before? This business of friends turning other friends off, then if those people are so easily swayed then I would agree with desparate that they are not friends worth keeping anyway. At the age of 17 they should be able to make their own minds up not listen to a poison mouth. So, if we are calling it bullying, would you DD be willing to get any help from college pastoral staff? I'm not suggesting a big inquest, just some support and advice, some one to chat to who knows the college scene.?
And leaving college aside as this is not the be all and end all, what does your DD do outside college? Does she belong to any clubs, churches etc? If she had something to distract her and another friendship basis it would be a help. Do you think she would be better with one or two decent friends rather than a big mix of not very solid friends, esp if she is quiet. I should have said before but I have a similar sounding DD who has had similar issues, although she is just 15yrs, she has found maintaining friendships a struggle, always. It has been a great source of help when she joined a sports class.
I have to go now but I'll be back on later, hope you feel better soon and you'll get some others on here with good advice. I'm sure your DD is lovely and she'll get some decent friends.