Search Google and read all that you can on why dogs attack sick or weaker dogs and how recognize, to deal with the issue before an attack occurs. I'd be involved in fun, rewarding obedience training my aggressor dog in daily short 2 - 5 minute fun training sessions x3 - 4 daily so she comes to recognize you as her leader, one who is always to be obeyed and who finds that doing so is highly rewarding for her, floods her with good feelings, oxytocins.
Teach her "Leave it" as a fun game so that she knows to immediately back off anytime she hears that command and that when she does, she will be instantly positively rewarded with a treat and your heavy, genuine praise, so that she'll be encouraged to quickly obey. Then should she begin to demonstrate body language that suggests she's fixating on the other dog, tell her to "leave it" and reward her lavishly when she does - a real celebration and over time, this shows her that it's far more rewarding to leave the other dog alone than to attack her from instinctive urges. Dogs can be trained out of their primal instincts.
I'd keep them separated until your weaker dog is fully recovered and back to herself, then slowly desensitize the aggressor to the weaker dog by only allowing them closely supervised, very short(2 - 3 minutes) socialization sessions, where you step and stand down the dog whose body language suggests she's fixating/alerting to the other dog and about to attack. Occas. dogs just attack without any signs so be sure you have the aggressor on a lead or wearing a soft muzzle the first few times only, then do not depend on the lead to do your work for you. Your aggressor dog needs to know that you, not the lead, have them under control. Have a blanket/heavy net handy should you need it at first.
01 Aggression: When One Dog is Sick - VeterinaryPartner.com - a VIN company!