She is the childhood friend, technically has dibs on the main guy
Nope, at least a couple of times over. Calling dibs is a thing
children came up with for an excuse to selfishness. Oh, we both wanted to ride shotgun? Well too bad, I called dibs. We won't even settle this with something like rock paper scissors, which is also somewhat childish, but at least somewhat fair to all parties involved.
Thinking anyone has dibs on another human being is probably a thing that shouldn't happen at all either, but if anyone has dibs it's the person who called dibs by actually making a move, so your second point about the thing she did wrong, that's at least sort of right. The other thing is, this guy had to consent to dibs being called on him or else it doesn't count then either.