sounds like you got a bad battery, any auto parts store can test it, they should be replaced every 4 years or so, so she should be getting to that point. if not test the alternator, agian can be done for free at any auto parts store.
after that i would say your leaving a different aplience on or you have a bad starting connection. aka check 2 make sure you glovebox light turns off, trunk light, mirror light ex...
if none of that your starter may have a bad connection, so it might not be a power problem but a wiring problem.( i dont think it is though)
but ya just go test that battery and alterbator.
good luck!