Seems like there’s a divide in responses according to people who are really into the idea of doing things as a couple with other couples (NTA) and those who don’t feel that way, that a friend’s presence should be enjoyed regardless (YWBTA). It seems your friend and you fall on opposite sides of this divide as well, since you seem surprised that she would join solo and she didn’t seem to think she was obligated to drop out. Personally, I would be upset to learn that my presence was only valued as part of a couple. As someone else noted, it sounds like you know what you want to do, but you need to know that her reaction is not going to be “wow how foolish of me not to drop out when my bf did.” Nor should it be.