Having too much of your credit in use at present can adversely affect your credit score, but I’ve never heard of high credit use in the relatively distant past having such an effect. They are talking about buying in 2009, so if they pay the car off now using a credit card it shouldn’t be an issue. As long as they stick to the plan of paying the card off right away (an important caveat), the rewards card approach should be a good plan.

I’d also suggest targeting any other high-interest debt for early repayment. Basically anything with an interest rate higher than what you would reasonably expect to earn from normal investments. That typically means credit cards, personal loans, and the like. If you have any loans with unusually low rates, such as student loans, it is probably better to invest the money rather than use it for early pay-downs.