So many things wrong here, where do I begin?
Okay, first thing you did wrong was a 100% water change. Never, ever do a 100% water change except in an extreme emergency (like if somebody yaks in the tank or something). Frequent partial water changes are a much better way to go. Getting rid of the filter media was the worst possible thing you could have done. Fresh water + new filter media = the tank is going to cycle again. Very hard for an already sick Betta to endure. It's hard to guess without seeing the fish, but I think I might use some Bettafix, or a broad spectrum antibiotic in her tank. At the very least, I would add some Bio-Coat, or something similar. (As a side note: Never use Melafix for Bettas, it's lethal to them.)
You should have performed about a 30% water change, thoroughly vacuumed the gravel, and thoroughly rinsed the filter media in the old tank water (never rinse it under the faucet, the chlorine will kill the good, essential bacteria in it). That would have gotten rid of the rotting food, and most, if not all of the excess ammonia.
Don't waste your money on test strips. They are highly unreliable. Everyone who owns an aquarium should have a liquid test kit. API Master test kit is a good one. They are about $20-$30, depending on where you shop, but the chemicals last a very long time, and after you buy the kit, you can buy refill chemicals as needed. They aren't hard to use, just follow the directions step-by-step.
If you are going to be away for a weekend, that's a great time to fast your Betta (should be done one day per week, anyway). It will not harm the fish to go without food for a couple of days. If she is healthy, it will be good for her. Feed her normally just before you leave and upon your return.
If you are leaving for more than a few days, show someone exactly how many pellets to feed her (and don't make it complicated with different foods. Just let her get along with her pellets until you return, she'll be fine). Write it down on an index card or piece of paper in case they forget. Tell them to pour the pellets out into their hand, not to pour from the package (that's how over-feeding happens). Best idea, don't trust other people, just buy an automatic feeder. They are relatively cheap for a base model, can only dispense what you put in them, and they won't "forget."
I have to ask, what sort of environment is she living in? It's very good that she has a filter. Is she in at least a 5 gallon tank, and does she have a heater? Bettas require a stable temperature between 78-82 degrees at all times. These are essentials for good Betta health and longevity. Does she have plants (silk or real, never plastic) and some sort of cave-like structure to take refuge in, or is the filter the only place she has to hide? Bettas (all fish, really) need some place to sleep and hide, or they will get stressed out.
I know you did not mean to make matters worse, you just don't know enough about aquarium keeping yet. What you should have done was a thorough tank cleaning, as soon as you discovered what your mother had done. A "thorough" cleaning does not mean start from scratch. It means a large partial water change, thoroughly vacuuming debris out of the gravel, and rinsing the filter media in old tank water. This would have gotten the excess food out of there before it began to break down and cause an ammonia surge.
What you must do now, is test the water at least once daily for ammonia, nitrites, and nitrates. When the ammonia level rises, do a partial water change with clean, room temperature tap water to which water conditioner has been added. Do nothing else! The tank has to cycle. After a bit, you will see that the ammonia level falls off and the nitrites begin to spike. Continue partial water changes, until that disappears. When ammonia and nitrites are at 0 ppm, or very close to it, and the nitrates are stable, your tank is done cycling. Continue monitoring the water for another week or so, to be sure. Then you can scale back to once weekly 20% partial water changes, testing, and light gravel vacuuming.
I strongly suggest you go to a library or book store and get some beginner's aquarium keeping books. Any half-way decent one will be chock-full of useful information that will help you as you learn how to care for your pet properly.
I hope she pulls through. They are very tough little fish, so she might. Good luck!