Urgent* Katerina isn't going in the litterbox

My brother found a stray kitten a few months ago. The kitten, Katrina, has been to the vet and is healthy. He has to find her a new home by Friday because she is not going in the litter box like she should. He lives in an apartment and the owner is complaining about the smell because she soils the floors. Katrina will potty in the litter box once, than she will not go in it again until it's cleaned out. So then she soils the floors. My brother has tried all different kinds of litter and litter boxes and nothing works. He needs to figure out something by Friday, or she'll have to be rehomed. :( Please, any help would be appreciated.

1. Has the vet been consulted about this particular problem?

2. Has he tried Cat Attract litter?

3. If she was a stray, could she find the litter box too small?

4. How often is the box cleaned?

5. There is a pet psychic here that has helped a lot of people. Also, a pet behaviourist may help, though I don't know how much they are.

6. Is she spayed?

Hope something gets figured out!

Has he tried putting more than one litter box almost side by side.
What he can do also, is get some dirt from outside and put it in the litter box with some of the litter. Once the kitten keeps going in the litter box, put less dirt and more litter each time. He might also have to keep the kitten in one room with at least two letter boxes, to retrain it to use the them. Once this is established, then the kitten can have free roam of the whole apartment again. I wish your brother luck.

I have three little boxes side by side as my guys will pee in one and then hop into another to make a poop. Plus I have three more boxes around the house.
Mind you I have 6 cats that use them. :)

Thanks so much for the advice. I sent my brother the thread and told him to check it daily. I have not had time to come on and see the thread until now. I have not heard from him to see if he rehomed Katrina or not, but I will update once I find out. Katrina is spayed, he cleans out the litter box daily, and the vet has been asked about it. The vet really didn't give my brother much advice except that she's healthy and that it hasn't got anything to do with her health. As for the other questions, I don't know. I'm hoping that he read your replies and figured something out.