I felt a bit sick, reading that article. The woman is clearly psychologically unhinged and her son will not thank her as an adult for being a "Norman Bates" style mother.
What if her ex remarries? Will the new wife have to hide in a wardrobe when his son visits dad, and pretend she doesn't exist? Absolutely deranged. What if SHE the bio mum remarries? I expect that would be different.
We can gain so much from a diverse number of caring adults in our lives (as kids). My son has a good relationship with his stepmum. I'm grateful for that. Life's hard enough....having one more parental figure who cares greatly for him, is a bonus as far as I'm concerned. It doesn't undermine me, our mother/son bond or my feelings of adequacy. I'm his mum. That's it. But my ex-partner's lady is welcome to love my son, and he her.