the reason i was thinking about velvet because of the silverfish line that he got recently. And since she is white i think it kind of difficult to spot. So i would not use light for now to make sure she don't have it. And keep eye on it make sure it not spreading
I am really don't think that she is lethargic because of the temp if she was fine in 78 i don't think that she got lethargic with 80 or even 82*. But it true that some bettas really don't like warmer temp and can get lethargic. I missed the point do you have a thermometer in her tank? Do you know what temperature she is in?
Also if you not home i would go to pet store and buy aquarium salt on your way home, in case you need to use it for her.
Since we don't know what is the problem i would do at least daily 50% for now. It will help in case it not just the temperature that is the problem .... i think more frequent water changes really can help a lot especially if the problem not the temperature.
I kept bettas in small containers for long time when i had to with daily water changes ....