No doubt the old battery is long gone, but six days does seem like a suspiciously short time for it to have drained. I had driven it 40 miles before parking it and plugging it in on Saturday, so it should have been fully charged at that point.I'm leaning toward it not being okay. To go from a voltage that was able to start the car drive it some distance (charging the 12 volt battery) then plugging it into charge (also charging the 12 volt battery), then go all the way down to 0 volts in 6 days means either there was a drain on the battery or the battery was bad. My guess is we will never know for sure if it was a bad battery since it was likely turned in for the core when she bought the new one.
The fact that it has happened before does increase the probability that there is some external drain when the car is off. This is something that you can check for.
I'll post again when the Bluetooth monitor is installed. Fingers crossed that it was just the old battery causing the trouble, because I won't even know where to begin looking for a different problem.