Like VIBE said, GF called with the intent of provocation. Stating "it's been decided"... signified to the BM that she's helpless and been rendered insignificant.
There must be some blatant or subtle antipathy between them or it could be a case of taking out unhappiness with the BD, on his relationship with another significant female in his life.
BM should have called BD immediately instead of getting belligerent in dialogue with GF and making her way with haste.
They both chose their course of action because they don't respect the BD and maybe even don't genuinely care about him. After-all, they're causing chaos and dysfunction in his life, in this case will also affect his child.
Better choices in women and arranging a meeting with the two females present at the earliest and also later on if there is clear ill-feelings between them would have helped avoid this or made it an unlikely occurrence.