well~ what I would do is cull her~
But your wanting to keep her as a pet.......so maybe what you might want to consider doing is.....

Put her on clean bedding with one other youngish female weanling rat for company. She shouldn't have to mess with who's "Alpha" if you use a weanling as her company. Keep her warm. Watch~ if she'll let you (BE CAREFULL~ this is going to hurt so she may bite you a LOT if she is not very tame)~ if she'll let you~ open the absess and let it drain. If whe won't~ leave her be. Her instinct will be to open it herself if she can. Go to the farm supply store~ pick up some teramycin powder~ mix a little in her water (won't hurt the pup~ but don't use the pup as a feeder until you've withdrawn it off the antibiotic for at least a couple weeks) If you can find some "Blue Kote" at the farm supply store spray that on the wound~ if not~ rub a little neosporin or something like that on it. Keep her clean, dry, and watch.

Good luck!