(and you have to admit there is a lot of UST between Brian and Mikey)
Well, this is my first and only fandom ever so I'm not a slasher. In fact, traditional slash makes no sense whatsoever to me.
So, I don't see *mutual* UST between Brian and Michael and I certainly don't see any chemistry there. I do think there's a powerful unresolved crush on Mikey's part and I think Brian plays along with it; uses it as a safety net but not meaning any of it.
I'd tell your friend to put down her magnifying glass and quit looking. There's nothing to find; those who ship Brian & Mikey are delusional or blind and she should go on and love Justin and ship B/J like the rest of us with at least one working eye and half a brain! ;)