Ideally she'd need to take her cat to the vet. The diarrhoea could be nothing to do with her over active thyroid or it could be a reaction to any medication she's taking. Usually when my cat has an attack of diarrhoea I get a Kaolin paste from the vets which helps to make her more regular. Changing her food might make things worse as it will take her gut a while to get used to the new food. Your friend could try feeding her a chicken and rice based food so it's bland and not too rich for her gut. But seeking vet advice would be my main advice. Sorry I couldn't help more. Hope she's feeling better soon.