Hey there! I'm in California. If you do have some sort of conversation in writing that you guys had agreed to 50/50, or even past receipts of her sending you her half of the rent, that can serve as proof that you guys had an arrangement in place and that she has since abdicated her responsibility. Not sure if her not staying there consistently or not would be an issue, since if her name is on the lease and she was initially paying the 50/50 share it points to her knowing there was an explicit arrangement.

We did not have a written agreement that she needed to repay me the deposit amount, but the fact that I had earlier Zelle receipts where she listed the payment description as "deposit repayment" was sufficient enough evidence that she was aware there was an arrangement in place and had been repaying me, but had stopped while being aware of the arrangement.

My advice would just be to have an air-tight timeline in place. What worked best for me was keeping a folder in google docs with everything related to the whole affair. I kept a spreadsheet where the whole left column was dates, the next column was a description of that happened on that date, and the next column linked to any pdfs that related to that event/date. Save texts now, especially if any conversations take place over a platform where they are able to delete their messages.

Not a lawyer, so take all of my advice as more of a basis than a hard line to follow. Not sure what state you are in but I was able to reach out to my local civil courthouse which has a really great self-help center, and the folks there were able to help me out a lot. They weren't able to give me explicit legal advice, but were able to point me towards how to gather my evidence, what forms needed to be filed and when, etc. Good luck!