Best friend slept with other best friends ex who she still had feelings for

Hi so pretty much I need some advice about one of my friends. So one of my best friends broke up with her boyfriend a few months ago because she wasn’t ready for a relationship but she still had feelings for him and he sort of did as well. Anyway a few weekends ago she really wanted to hook up with him and it was almost certain to happen because they were all over each other all night and my best friend was voicing saying how much she liked him to me and our other best friend. So pretty much I found out that our other mutual best friend ended up sleeping with my friends ex on that same night after our best friend had been saying how much she still liked him. The mutual best friend sleeps around quite a bit (which is completely fine no slut shaming here) but is this where we draw the line because she took advantage of the fact that said ex would sleep with her and completely disregarded our friends feelings. I just need advice as to whether or not I’m being to harsh because I feel like she completely broke girl code.