If you cannot gift the money without resentment, do not co-sign. Their default becomes your credit crater with zero upside.
If you cannot gift the money, do not co-sign.
Other Truths you may like to hear
Money
Your salary is a drug
Own assets or stay rented forever.
MoneyHouse-rich cash-poor is a trap
Do the math; sometimes renting keeps you free.
MoneyDebt loves optimistic forecasts
Stress-test payments at unemployment rates.
MoneyA car is a depreciating safety box
Buy reliable used; invest the lease difference.