"Charge-off" means 90-180 days have passed since she stopped paying, and the creditor has decided payment in the near future is unlikely. It's a tax status for them. However, it does not erase the debt.
You need to pay for the car
You agreed to co-sign so you are responsible for paying for the car, unless you signed while not of legal age, or were coerced.
You knew that when you co-signed, and if that resulted in some temporary peace in the family, now is when you pay for that. You knew the score. So it's time to adult up.
I gather by the fact that the debt was "charged off" that the car has already been repossessed. Now it just comes down to money ($$$$). They will have sold off the automobile, reducing the debt somewhat, and you need to pay the remaining debt.
Contact a lawyer or your local legal aid. Have them negotiate with the company that you will pay an amount to settle the matter in full. A condition of the deal is that the debt will be completely and totally removed from your credit report, and neither of you will disparage the other in any way including credit reporting. Most creditors will agree to this if it means getting paid. Also I recommend adding a condition that you agree there is a genuine dispute as to the validity of the debt, and so, any amount they consider forgiven will not be treated as income. That will keep you from paying some taxes.
The lawyer can dot all the I's and cross all the T's.
Wait until both you and they have signed the contract before sending any money.
Then you write a fat check and you're free and clear.
If you don't pay
Then the creditor will hold onto the debt for awhile, on the off chance you do decide to start paying. After awhile, it will "sell off the debt" to another company. That company will make collection efforts against you. After awhile, they will sell your debt to another company. They too will make collection efforts against you. This could go on for 20 years, although after 3-6 years they will lose the right to sue you for the money.
Your best outcome will happen if you settle with the original creditor. Once they sell the debt, that becomes impossible, since they don't own the debt anymore. Therefore you lose the ability to haggle with them to stop reporting it to credit bureaus. It's on your credit report for good at that point.
By "for good" I mean for 7 years after the last payment.
Yes, it will suck to find an apartment during that time. Your best bet there will be "housemate share situations", which are a cramped lifestyle, but at least you are homed, in the locale you desire. If you are crafty, you can also consider Tiny House on friends' property, or buying cheap land and building a Tiny House... or even #VanLife.