I don't think this is double genderswap.
I think the childhood friend was saying goodbye to MC because she was returning to be a hero in her original world and would never see him again.
So the description should look more like this.
The MC's childhood friend (♀) suddenly shows up to say goodbye to the MC (♂) after vanishing for weeks.
Childhood friend (♀) gets attacked by a living weapon (♀), but the MC (♂) gets in the way and dies.
The MC (♂) is then reincarnated in another world as the living weapon (♂->♀) that just killed him created by the demon lord, while his childhood friend (♀) suddenly shows up as a hero vowing to fight the demons.