Bitchy Bridesmaid

Hi I am getting married in May and one of my Bridesmaids was my roommate that I THOUGHT I became friends with. Anyway she came to visit and just bought her dress for the wedding, but she’s friends with a current roommate that does not get along with me. Well it turns out they are constantly complaining/talking bad about/making fun of me together. I want to kick her out of the party but I have really bad anxiety and can’t help but feel bad because she already bought the dress. Any advice??

UPDATE: I told her and she tried to deny or put down any of the things I said, but long story short, she’s out and I am happy! Thanks everyone for the support!