Idk how many times diapers have been suggested, but diapers for sure!! How big is she? I find that size 0 diapers fits pretty good and you just have to cut a hole for her tail. OR you can buy cloth/reusable pet diapers and just stick a pad in it
Seriously it will help so much and you will be able to keep her! She is 5.5 and a little older to start understanding pottying outside, so diapers would be great.
You could try pee pads since they stay inside.