Do you think it died of neglect or old age, or is it perhaps a disease?
There are methods to clean a tank safely to make sure nothing will be passed from one fish to another. Diluted bleach is used but Its tricky and there are other, more natural ways. I dont know which kind it is, but vinegar and water is a great option, too. Also, I think canning salt can help, too.
If you want, post a thread in the habitats forum and ask them there. You'll get good answers, as many members have done the same as you. This way, you can make sure your friend not only gets a nice, new home to play in but that he remains healthy to enjoy it!