Ok, dropsy is a fungus, and fungus is HIGHLY contageous. If you have a hospitol tank, drop it in there. Get it away from that fungus. Some people say salt is a deterent, but when my fish had fungus problems, it only made his condition worse. Put it in HT, and tomorrow go get some fungus eliminator from Jungle. Yellow powder, clears up in hours.
*not to scare you, but if you let the disease get so far that she's breathing heavily near the top of the tank, all you can do is pray that she lives till tomorrow.*
Also, is it in the guppy's gills?