Female guppies are prone to
Flexibacter columnaris, commonly known as cotton mouth and/or can be associated with fin rot. The ich probably weakeand the immune system, leaving the guppy susceptible to the disease. I would either isolate her, or treat the whole lot of them since columnaris is very virulent and will quickly spread if not contained.
You need to set the heat to 74 degrees farenheit, since anything above allows the disease to reproduce too fast for the medicine to take effect, and too low won't let the medicine work. Then, I'd treat them with Mardel Maracyn Plus, or a combination of Maracyn and Maracyn-Two. Adding aquarium salt and almond leaves or blackwater extract will help the fish fight the infection. Salt kills the bacteria, and the leaves excrete an antiseptic like property. It also lowers pH, which the bacteria hate. Be sure to have adequate oxygenation, as the medicine reduces oxygen in the water.
I'm currently dealing with a betta with columnaris, it's nasty, but he looks like he'll make it
It's the best thing to act quickly and cure it as fast as possible.
Try feeding the fish either garlic rich foods like New Life Spectrum +Thera-A, or soak their food in garlic juice (either homemade or store bought Garlic Guard by Seachem) I have a technique to make homemade garlic juice if you'd like
Garlic gives fish a healthy boost to the immune system, helping them fight back.
Best of luck