What has caused my car not to start?

Hi
Basically my girlfriend has bought a ford fiesta. 2004. She wants to learn in it so I took her to a car park and supervised her for an hour or so. She stalled it at most 5 times but generally very good. At the end we talked for about 15 minutes (with the headlights on) When it came to starting the car back up it cranked but didn t start. It sounded normal speed and tried it for about 5 seconds. Tried it a second time and the battery started to get really weak. After a third time it went dead. Did a bump start and it turned over and started just like normal.
What would have caused this?
She did mention she had been listening to the radio for about 25 mins during the day but when I first started it up and traveled 2 miles with it to the car park it started fine.

Anything to worry about?

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