In fact, traditional slash makes no sense whatsoever to me. I was that way too actually. I remember the first time I heard about slash (Lord of the Rings slash) I was kind of disgusted and disappointed (I mean can't two people be *friends* without sleeping together?). Then I got into QaF where it was canon, so it was ok, then when I moved onto other fandoms, well, I found I just couldn't read het anymore, but was too much of a romantic to read gen, so slash it was. :P
So, I don't see *mutual* UST between Brian and Michael and I certainly don't see any chemistry there. Ok, point, I don't think it's mutual UST, but I do think it's mutual chemistry. They both have a co-dependence on each other. Sure Michael's the one that's hot for Brian, but Brian isn't completely uncaring to him. Michael, right up until the very bitter end, was the *only* one Brian would admit to loving. Brian *needed* Michael. And we see that in season 5 when they have their falling out, or even earlier on in season 1 when Justin has to get them back together.
Look at Brian's jealous rages. How he works so hard to break Mikey and Dave up. How he tells Mikey not to fall in love with Ben. And even though he didn't outright sabotage Mikey and Ben, there were quite a few instances where he did it subtly, like when he kept dragging Mikey around with him, keeping him from Ben. How he tries to talk Mikey out of marrying Ben, and then criticizing how he's changed once they are married.
Brian even offers to sleep with Michael more than once throughout the show, and I think he would too, because as much as he doesn't want to go in that direction with Michael, he'd do it if it meant not losing him.
People often look at Mikey and Brian, and see Mikey crush, but what they fail to see is that Brian needs Mikey way more that Mikey needs Brian.
I'd tell your friend to put down her magnifying glass and quit looking. I don't actually have any friends who ship B/M. I don't really get it myself (as much as I just fought for it, I can see the chemistry, but I wouldn't want to ship them because I don't think they'd ever make it as a real couple), so mostly hung out with B/J shippers while in the fandom.
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! ;) I don't see anything wrong with shipping them. The fandom is big enough that they can have their corner, and we can have ours. *shrugs* I mean B/J shippers have an entire *archive* so they really can avoid it. Just think of all the poor B/M shippers out there who have to put up with not only canon in their way, but also B/J shippers at every turn :P