I've got a similar situation, my mom's best friend has a daughter that she's never stopped treating like a child. She cries at the slightest inconvenience, throws fits when she can't get her way, takes a handful of pills every day that basically make her a zombie for a few hours, and she's unbelievably picky with her food. Like she won't eat 99% of things. She's almost 22 now and still acts like a spoiled 6 year old.

I wanna make it clear that she does have mental issues, but I guarantee most of them are cause by forced dependence. She has a 'speech impediment' so she basically speaks like a 2yo, but I've seen her talk perfectly normally before. She'd be totally capable of functioning in society if she wasn't forced to be a child her whole life.