Should we pay cash for a new car?

Ok, my fiance' owes the irs 20k, he also owns 2 llc's both operating at a loss, he just cashed in a bunch of stock, now he wants to pay $40k cash for a new car! I told him now that he has some money in the bank he should pay off the irs, invest the money in tax sheltered investments and lease a car for 2 years and write off the in lease payments through his business. Someone help me convince him not to put all that cash into a new car!