Vehicle starting or possible electrical issue

This vehicle has become a back-up vehicle due to the wife of the person I am working on this for, is no longer able to drive due to a brain tumor.
The main issue that was going on was the battery was dead, so put charger on it to charge battery. The person drove it for a week. Let it sit for four days again the battery was dead. Chose to take alternator out to get tested (because when in vehicle it was testing fine) and alternator was bad. Changed that six months ago and it was running great. People would ask to borrow the vehicle so it was being driven more. Went out to start it after about three weeks of it not being run, only clicked. Jumped the battery and drove it around for twenty minutes. Again about an hour later, it was dead. Today the owner put a battery charger on it and it said the battery was fully charged. The interior lights and dash lights came on. Turned key and click. After a few times of turning key and hearing click, the interior lights went off and dash lights went off. I am at a loss with this. Tried both keys that worked with car and tried to jiggle keys. Nothing. Any help would be greatly appreciated. Thanks in advance.