Get an order for the visitation rights for your son and leave her to grow up and fend for herself.
She is obviously very immature.
Don't feel guilty. She is who she is so take care of you and her son. Let her parents co sign on a car for her. You have no other financial obligation to her but support for your son.
She does not sound to responsible as a mother so I'd keep my eyes open on that front as well.