I've seen my brothers selectively identify the type that would betray a friendship -
I am not that type of chic and would be insulted that a dude even thought he could pull that w/me.
I completely understand this in terms of how someone thinking that of you would be insulting. What I do not understand is how that's betraying the friendship if she was straight up about what happened and asked for the green light before taking action, and what I do not understand is why the friend would think it's a betrayal.
Me personally, I'm of the opinion that it's on you to make yourself happy. And I don't think anything should be allowed to stand in the way of that pursuit. If the friend who is on the

isn't actively putting the other friend on other cats ... how she feels about any of that is inconsequential, especially if she didn't want him like that.
Now, if they put time in, love was built, etc. then yeah, she gotta fall back. I don't think anything worthwhile was built to where other chick should have to fall back.
That said, friend should have caught the red flag as soon as the other chick was like, "Don't leave him. He's a good dude, give him a chance, etc." because USUALLY, if a woman is thinking about dropping a breh, her girlfriends will ride on that and probably talk about the other cat(s) they have lined up for her. What that tells me is that she had been eyeing him from the jump or at the very least, saw the potential to where she felt he'd be better off with somebody who would better appreciate him (like herself). She had been thought about it.