<human>: Could you distill the essence of the text into 1-3 sentences?

My best friend has been seeing a really nice guy for the passed couple of months or so, and they were doing great. She has a 3 year old and her guy and her kid got along great, they did a lot of stuff together (go to the park, go on walks, movies, play dates with friends) and she saw potential in the relationship. About a week ago he just randomly stops talking to her, stops messaging her or responding to her messages claiming he's just busy. 

Now, I've met this guy only a handful of times, a few times when he came into our work (my best friend and I work together) and another time when I was at her house and he came over...

She asked him about a week into him ignoring her what's going on, and if they are even going to see each other anymore, and he never responded. She just decided she was done with it and it was time to move on.

I open my "other" folder on FB (I almost never use my FB on my computer, I usually only check on my phone), and there's a message from him confessing that he has feelings for me and not her, and wants to meet up and talk!!!! I'm absolutely blown away, and he asked me not to tell her. She's on her way to work right now, so I don't wanna ruin her day, but how should I approach her with this? I'm obviously going to tell her, she's my BEST friend and he's just some random guy I barely know (and he's too old for me) and I'm not even attracted to him, I'M SEEING SOMEONE! Should I message him at all telling him how I feel about this? Or let it be? What should I tell her? She has a bit of a short temper and I don't want her to freak out. She might get really upset, but I HAVE to tell her.
<bot>: BFF's guy MSG'd me on FB asking to meet up after ignoring her for a week. Let it be or tell him off on behalf of her? How to tell her about it without her freaking out?