My best friend told me something in confidence and now I'm watching it actively hurt a third person. What do I do

So about two months ago my best friend told me she had developed feelings for our mutual friend's boyfriend. She wasn't acting on it, she just needed to say it out loud to someone she trusted, and I get that. I told her I appreciated her being honest with me and we kind of agreed it was just one of those things that passes and left it there.

The problem is it hasn't passed. I can see it every time the four of us hang out. She finds reasons to be near him, she texts him separately about things that don't really require a separate conversation, and last week our mutual friend made a comment to me about feeling like something was off but she couldn't put her finger on it. She wasn't accusing anyone, she just seemed genuinely unsettled and sad about it. I sat there and said nothing because what am I supposed to say.

I'm not going to tell our mutual friend what I know, that feels like a betrayal of the confidence my best friend placed in me. But I also feel genuinly uncomfortable pretending everything is fine when I can see someone I care about being slowly unsettled by something she doesn't have the full picture on. I haven't brought it up again with my best friend because I don't know how to without it becoming a huge thing. Has anyone been in something like this and actually found a way through it that didn't blow everything up.