Car wont start???

Ok I had to jump start my car yesterday and i used my wifes car to boost mine. Now mine starts fine but her car wont . so I gave her a boost , and we kept the car on for 20 mins afterward. Then later today i tried to start her car and it was dead again?
whats wrong with her car
her car is a 89 civic sedan.
Please help cuz i need to know asap.
Thanks

Alternator. Put a multimeter on the battery, if it ain't reading above 14 volts, goodbye alt.

was her car ON when you jumped your car? if not then u drained her battery

well if her car will run once it's started and the lights aren't dimming and the car dying its prolly not the alt... check your gound and make sure its all happy like first off... and then if thats all good my next suspect would be the battery, how old is it? i'd have it checked out... if its none of the above let us know