Betta Fish: How can I help?

Hi. My friend sent me a picture of her betta fish that she has had for.. a few months now, but i fear he won’t make it much longer. His colours are very dull, he has no plants, painted gravel, murky water (usually) and he seems to either have finrot or has been nipping at his fins. His eye also looks weird in the picture? I don’t know how to help this poor baby! I only have one tank— and it’s currently home to my male betta and the quickest I could purchase a new tank is a week. Then it’d have to cycle… I’m not sure how to go about this.