Her tank was below 70 degrees? That is VERY cold for a betta. Temps that cold can kill them. Do you have a heater for her tank?
If it is pop-eye, Bettafix is not likely to be of any benefit and can cause the fish breathing problems because it can coat their labyrinth organ.
True pop-eye (which can also be a symptom of dropsy) needs a very different treatment. Can you take some pictures?
For now, give her 100% water changes every day and dissolve *epsom salts* (not aquarium salt, and make sure it has no scents or additives, just plain epsom salt) into her water, 1 teaspoon/gallon, which can be increased slowly up to 3 teaspoon/gallon.