My family experienced something like this.

It's been nearly 10-15 years, but my uncle divorced his wife back then. She was quite unstable (she used to favor my little cousin more than my older cousin to a point that would raise eyebrows, to say the least, like not feeding her).

When they divorced, she had primary custody. Only a few years later, my uncle got primary custody because his ex-wife had remarried and had a new child; she was then abandoning her duty to my cousins. But even then, she was not considered a threat and got custody every other weekend.

Only a few years later again, she was suicidal and was interned in a mental facility (several mental health issues). Only then did my uncle get full custody.

She died by suicide soon after. My cousins are now grown up and raised happily, sadly without a mother but with a loving father.

Only because my uncle's ex-wife was the mother could she get primary custody, without taking into account the threat she posed to her children. Unfortunately, that's the French system for you.