I was spanked as a child as were my two sisters, I know because my mother told me so, not because I have vivid memories of being spanked. I do remember my sister getting spanked but she was 6 yrs younger, so I was a teen when she was being corrected. My point is spankings don't have I be a life trauma to work. It's meant to give a physical response to coordinate with the verbal.
I don't have kids so all I can relate to is my dog. Sometime I can correct her actions verbally till I'm blue in the face but a small pop on the butt and I've got her attention. And I can attest that I would never hurt her and would fight anyone who did so, unequivocally.
Unlike dogs children have the ability to reason, that's why spankings in most cases, as previously stated, can be rare.
I can tell you this, I've seen kids spanked in a manner that was not abusive, but that I strongly disagreed with. A kid just being a kid, and the parent was annoyed or in a bad mood. And on the other hand I've seen too many kids that need a spanking but will never get one.
To the OP if you live in the house all I can say is be vigilant and look out for the child the best you can. Sounds like the mother is beyond talking to.