I had one guppy have a similar thing happen where his dorsal (and a pectoral fin and a bit of tail) seemed to 'melt away'. It sounds similar to your case, but he had a white patch that looked fungus like around the missing fin and swam like he wasn't feeling entirely well. Unfortunately, he died a day or two after he lost is fins.
I would say, unless she seems like she is in distress, or has other symptoms of a disease. Continue treating with stress coat and aquarium salt, and most importantly make sure to keep the water clean. Keep us updated on her condition.
I would say, unless she seems like she is in distress, or has other symptoms of a disease. Continue treating with stress coat and aquarium salt, and most importantly make sure to keep the water clean. Keep us updated on her condition.