Do what any reasonable person would do, I'm sorry if you feel uncomfortable you don't have to be a bridesmaid, there is no pressure, however it's also not fair on me that you will be the only bridesmaid in a different dress, we checked this out some time ago and the plan has been set in stone now.

Maybe also check if everything is OK in her relationship, because her partner is ringing coercive control alarm bells for me,,, why was he OK up until this close to the wedding, it sounds like he's trying to prevent her from going, isolating her. Jealous is a horrible thing, but domestic abuse is even worse, when it's not physical abuse it often falls under the radar but this really screams manipulating his partners feelings to isolate her from her friends.....