The mother was under immense stress during the flashback.
We can see that her hair has gradually turned white. Pure black when meeting the doctor, black and white when meeting with the spiritualist, mostly white before her end, and pure white after.
On another note, Keitaro's comments about letting his guard down really make me think that every time they visit a new rank, he is always the one who suffers the most.
First time in an S-Rank haunted spot, he gets separated from Yayoi and meets the full brunt of H Castel's ghost frenzy.
First time in an A-Rank haunted spot, got kidnapped, drowned, and almost became a human spiritual battery.
And now, the first time they are in a B-Rank haunted spot, his spiritual body gets separated from his physical one, and his soul gets taken away.
He really is a tasty delicacy for ghosts.