Well here's my thinking on this:
If the tablet can't be "revived" when it turn dark, why would charging it help?
Two scenario where charging alone would bring back a machine:
- machine was out of battery;
- machine runs into problem, battery drains on black screen, then when you plug in again it gets rebooted, which might solve the original problem.
However, if your mom plugs in the machine soon after it turns black, scenario 2 would not occur.
If your mom has been using the machine all day however, scenario 1 might be the case. Surface's ideal battery life is about 8 hours, typically shorter if continuously used with high brightness, volume, and other battery intensive apps (some games use a lot of batteries). The amount of apps that makes the background tasks running might also be an issue. How strong is the wifi-signal in the house? Lot's of batteries can be used up when machine tries to search for better signal. Also, was the machine really fully charged every time?
On the other hand, If the issue is not reproducible, as in, truly random, no notification/warning at all and can't be demonstrated on demand, it's likely a faulty product.
Since no one else here seem to experience that either, this might be an unique case. Best Buy might had it mislabeled when it's actually refurbished. I have always thought the battery low warning on all different versions of Windows a bit annoying, as in, they usually start kicking in like at 20%. But if you got no warning at all, that is very very strange. Was the screen dimmed automatically before it shut down?
In any case, it just seems weird to me that charging it over night would fixes (temporarily) the issue, since nothing has been done to the tablet at all really. It wouldn't really make sense to be a software issue here (because then you should be able to reboot it on the spot). Take it to the Microsoft store if there is one near you actually, Best Buy doesn't always have the most competent people running these things.