did she step out of line?

my boyfriend has a friend group that he hangs out with - we go to different colleges so I’m not apart of that friend group and because of covid I haven’t met all of them yet. We have been together for one year. this happened last weekend and he called me as soon as it happened after. I got so mad at the girl. this is what happened:

he was driving his two girl friends to his guy friends house where they would play cards. one in passenger seat and one in the back. the girl in the passenger seat says “I have to tell you something” and then confesses that she developed feelings for him. he said that he reminded her that he was with me and didn’t feel the same way. she said “I just had to tell you and get it off my chest”.

was it wrong for her to do that? I personally would never confess my feelings for a guy if that guy had a girlfriend at all, much less for a year. it felt a little homewrecker-y to me.plus her friend was in the backseat, isn’t that awkward? my boyfriend said he felt trapped and awkward and he hasn’t talked to her since. did she step out of line? is this weird to anyone else? I know she knows about me cause he told them about me and she followed me on instagram a long time ago.