She needs to go back to therapy as it's clear she is dealing with a lot. I don't think someone wearing a headdress 10+ years ago when cultural appropriation was talked about is a reason to deny your right to go see your bestest friend get married.

Of course she can decline to come, but it doesn't give her the right to dictate to you that you can't go. Has there been any other instances where the bride or groom has said anything remotely racist and it could feel as if you are taking their side over her? That's the only thing I could think of that might upset her?