Good that you have caught it early! Don't separate piggies if you can avoid it.
If her breathing is not quite easy, place a bowl of steaming water next to the cage, and keep that refreshed from time to time. It helps to ease the breathing, make her feel better in herself and keep up the appetite. You can add a drop or two of olbas oil (NOT vicks) if you really wish to, but just the added humidity will be totally fine, especially in this cold weather when the radiators are on full.
Also give her a pinch of probiotics either 1 hours before or 1-2 hours after the antibiotic to help bolster her guts. If you wish, you can also do a 2 week booster course of vitamin C to help strengthen the immune system.
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
As you have caught it early, there is every reason for a good and quick recovery.