Manx kittens & cats in Fillmore, California

I took Sol in a couple months ago from a close friend that was moving and couldn’t keep her. Everything was fine and she got along with my 4yr old cat, but would bully my very active, playful 4-month old kitten. I tried my best to introduce them the correct way, but I have a feeling she would mistake his playful advances for malicious ones. He would avoid her and stay away and I thought okay this might work, but as he grew and got older they became even more distant and recently had a fight. Ever since then she barely comes out unless it’s to eat, and has stopped using the litter box even when I tried leaving it in her designated office space room that she likes to sleep in. I took her to the vet and no UTI or other medical issues, but most likely a behavioral issue. I’ll be leaving the country for a couple weeks soon and I don’t have the time to try to help her. I don’t want my friend who will be cat sitting to have to deal with cleaning up accidents either while I’m gone.