I think you are barely paying for a nickle more than food for your child and it's a tight squeeze at that. So this single young women is footing the bill for housing, food, insurance,clothing, medical expenses, the daily living costs, for your child, of all sorts.
You don't owe her a car. But it's ridiculous to suggest that she's mismanaging the tiny stipend you supply because it isn't covering more! Ridiculous. You are insulting yourself by even talking like that.