Unsure what to think of my 29F partner’s relationship with her 28F friend and if it crosses any boundaries.

My 29F GF is very close, essentially lifelong friends with her 28F friend. They are both very open about sex with each other down to details about sexual encounters. My partner is bisexual and the friend is likely bisexual as they have hooked up with women before in group sex. In the past they have been physically intimate and have more than once made out at parties (this is the extent of it AFAIK). I know people kiss their friends when they’re drunk but this was much more intentional and intimate than that. The last time this happened was a few years ago and finding out it wasn’t the first time was pretty upsetting to me and we talked about it. It hasn’t happened since but the other day I was next to her and noticed an exchange of bikini pictures with seemingly no context other than “look how good I looked in this swimsuit” type of thing. So now given the history I explained i am pretty uncomfortable and voiced my concerns about it. It basically ended up with her reassuring me that they agreed they would never have sex or hook up with each other because of their friendship, which left me curious as to why this was even brought up between them. I don’t know, I’m a guy and I get that girls are much different when it comes to this type of thing, IE maybe giving a smooch when they’re drunk or sending clothes try ons, but everything together just doesn’t sit well with me in this case. How does it look from the outside? I trust my partner but wonder if there’s some boundary she doesn’t realize she’s crossing or something?