So he parents making her pay for the car in my eye's regardless of over price this is them trying to force her to look for a job to pay for things without telling her to do it they obviously see that, She need the money and don't want to coddle her with having to pay for everythink hence her having to pay rent, eventually she will click and she will do exactly what she needs to but it's really down to you to work out how long you can wait for this to happen because it could be years before this clicks and by that time she will have debt she will keep from you becuase she will believe in her mind your always going to suport here and help, and thats strictly not true 100% of the time, your not married so you have no obligation to financially support her. as for you feeling there not supporting her they are supprting her therer trying to make he grow up and support her self which i am sure your parents did to you but at an earlier age which they didn't do til it was to late as,
As for talking to her about this, my suggestion is you don't just let this go because it a year or 6 month time this will make thing 10x worse on both of you and by that point it really will be to late, you really need to have a serious conversation about this together and try together to listen if not then horrible as this may sound, right now is not the time for you to be together in life, but thats truly your choice, but she really does need to learn she need a part time job to survive just like you,