I think people naturally DO rp with people outside their friend group. Most of my oldest friends are long since tired of rping with me. I think most players really appreciate finding someone ‘new’ and fresh to rp with, and it’s the default state to search for that.
But when it comes to ooc trust, they are gonna rely on those they are friends with. When their characters are threatened, they will turn to their friends for support before a stranger in an IC position that might make (more?) sense. To a lesser degree, they are also going to disseminate plot to their friends. A lot of plot handed out to players is "X is happening, get together a team of pcs to address/experience it’. A lot of people will default to known quantities! It makes sense. I think a player maliciously or intentionally excluding others is much rarer than might be assumed.
IDK how to change it or even if you should try, but forewarned is forearmed or something.
I’m so glad she had a good holiday, and you got to see it