Peanut's rotten tooth :(

Peanut has a rotten tooth! For those of you that don't know her, she's a very old cat that was left behind in a loft by a previous owner. After the loft got bedbugs and every one moved out, we ended up with her. We've only had her for 6 months but love her even though she pees over the edge of her litter box. If you've ever had a good time at a loft and there was a cat with a tiny head it was probably peanut. We have to save her but we can't afford it! Please help, we and everyone else love her very much.