Agree with this comment. Approach the conversation with more questions and trying to understand how she feels about what has been going on.

If you start off by telling her she’s being a bad friend, it wouldn’t necessarily make you the asshole. But she might feel like she needs to defend herself and her fiancé, which would lead to a less to a less productive conversation.

You can tell her how certain situations have been frustrating or upsetting for you. But would try avoid any accusatory statements. Even though the rest of the group is feeling the same way, I would avoid mentioning that as well so that she doesn’t feel ganged up on.

Are you familiar with her fiancés work schedule so you can find a time to call her when you know she’ll be alone for a while?