but is it OK to go to an interview in leggings and flip flops?
It isn't a senior role and there's no particularly formal dress code, although actually leggings are not allowed and flip flops are banned for safety reasons, candidate would not necessarily have known that.
I know what she looks like shouldn't make a difference and maybe it's all she had etc, but surely it's still normal to make some sort of effort to be smart for an interview?