If the battery keeps dying, you need to find out if you have a short circuit somewhere. Find a friend with a digital multimeter and go to an electronics store and buy a 20 ohm resistor. Remove the positive battery cable and touch the resistor to the cable and the terminal and read the voltage. In electricity, V = I R, where V = Voltge, R = resistance (you can read the value with the meter. it will NOT be as marked), and I = current.. So, current = V divided by R. If there is not a short, you should have some small current ... on the order of less then 0.100 A or so. If you see more than that, you have a short somewhere. Many cars have a car-off current of .01 to .02 A (10 to 20 mA).
I'd have him read the current directly but, if it exceeds 10 A, it can kill the meter ... and we already suspect a short circuit.
If you have a short, it is a matter of troubleshooting it. Since you are asking in here, I am assuming you do not want to troubleshoot it. If in warranty, have the dealer look at it. If not, your call.
If you don't have a short, you have had a succession of weak batteries.