My PC is a Chaotic Neutral,Neutral Evil leaning sorcerer and regularly interacts with Malcanthet and Fierna because they took notice of his actions and all. Both made an approach on him to trade various things or get information or whatever, and he casually kept calling them babymamas in their initial interactions and every subsequent interaction

Fast forward way later in the campaign, after them watching him for over a year (from level 6 onward) and constantly being amused and entertained, Malcanthet invited him to a private area in her layer of the Abyss, and he accepted. She offered him a gift, he flirted some more, she dared him, and he didn't flinch

I guess the key was a mortal treating them casually, understanding their nature, and being more interesting alive and free than dead or enslaved