Separate the frogs. Give the sick one a bath in some salty water for about 10 minutes once or twice a day (2-3 teaspoons per litre/two pints). That should kill the fungus within a few days. Don't leave the frog in the salt bath for more than 10-15 minutes each time, because the salt will start to damage the frog's skin. Of course, this is all useless if the animal is going to get attacked again (maybe you are not feeding them often enough?). So you need to make sure this doesn't happen again.