I guess I could just use some advice

my partner and I been together for a few years now and we recently opened it up at the beginning of this year

she started dating my best friend, making him my meta

honestly, I was slightly bothered at first but I put myself in her shoes and wondered how she'd react if I did the same thing. and also considered that being bothered is ultimately a stupid thing to do considering how much they both mean to me, and I have a massive crush on one of her associates.

he broke up with her today on the eve of a massive milestone in her career and now she's devastated

I know she still loves me more than anyone and I know I will always put her first, but I don't know how to navigate this

one's the woman I intend to marry and the other is my best friend that I've known since college, and have always wanted as my best man someday

I just need some advice on how to help her get thru this heartbreak and how I should approach my friendship with him