My neighbor’s betta

I’m house-sitting for my neighbor and I feel terrible for her fish because I did some research and I think it’s extremely neglected. There’s no filter or heater and I asked if she had water conditioner and she didn’t answer that part and said you don’t have to change the water until next weekend. I bought some anyway off amazon which arrives on monday and I changed 50% of the water and removed some of the old food. I want to offer to take care of it so I can save it because she literally said she’s waiting for it to die. How much will it cost me because I still live with my mom and she probably won’t be happy anyway?